Fix type checker errors (#12081)
Fix type checker errors (#12081)

This fix the following errors thrown by the type checker when the
FormDataWrapper class starts to be used:

src/form-data-wrapper.js:79: ERROR - Property entries never defined on FormData
    if (!this.formData_.entries) {
                        ^^^^^^^

src/form-data-wrapper.js:101: ERROR - actual parameter 1 of Object.keys does not match formal parameter
found   : (Object|undefined)
required: Object
    Object.keys(this.fieldValues_).forEach(name => {
                ^^^^^^^^^^^^^^^^^

As well as fixing the nullability of the return value of `getFormData`.
It should really be non-nullable so that callers don't need to check
for null.

Also improved the type safety of the fieldValues_ variable to have the
key and value types.
honeybadgerdontcare/amphtml,lannka/amphtml,jpettitt/amphtml,jmadler/amphtml,zhangsu/amphtml,koshka/amphtml,laterpay/amphtml,jridgewell/amphtml,yieldbot/amphtml,jridgewell/amphtml,zhangsu/amphtml,taboola/amphtml,luciancrasovan/amphtml,honeybadgerdontcare/amphtml,kamtschatka/amphtml,cory-work/amphtml,chaveznvg/amphtml,voyagegroup/amphtml,yieldmo/amphtml,cvializ/amphtml,smartadserver/amphtml,clawr/amphtml,rsimha-amp/amphtml,mhc-gh/amphtml,shawnbuso/amphtml,bpaduch/amphtml,sharethrough/amphtml,alanorozco/amphtml,nexxtv/amphtml,zhouyx/amphtml,zhouyx/amphtml,koshka/amphtml,src-code/amphtml,prateekbh/amphtml,chaveznvg/amphtml,laterpay/amphtml,powdercloud/amphtml,sriramkrish85/amphtml,yieldmo/amphtml,prateekbh/amphtml,zhouyx/amphtml,Gregable/amphtml,camelburrito/amphtml,BenDz/amphtml,jmadler/amphtml,BenDz/amphtml,zhangsu/amphtml,zhouyx/amphtml,jpettitt/amphtml,sriramkrish85/amphtml,jridgewell/amphtml,danielrozenberg/amphtml,luciancrasovan/amphtml,techhtml/amphtml,tlong2/amphtml,cory-work/amphtml,tlong2/amphtml,yieldmo/amphtml,src-code/amphtml,smartadserver/amphtml,sebastianbenz/amphtml,media-net/amphtml,jpettitt/amphtml,lannka/amphtml,rsimha-amp/amphtml,comScore/amphtml,alanorozco/amphtml,ampproject/amphtml,erwinmombay/amphtml,jpettitt/amphtml,techhtml/amphtml,adup-tech/amphtml,dotandads/amphtml,cvializ/amphtml,honeybadgerdontcare/amphtml,Gregable/amphtml,tlong2/amphtml,yieldmo/amphtml,luciancrasovan/amphtml,alanorozco/amphtml,widespace-os/amphtml,aghassemi/amphtml,kamtschatka/amphtml,zhouyx/amphtml,prateekbh/amphtml,engtat/amphtml,yieldmo/amphtml,engtat/amphtml,donttrustthisbot/amphtml,yandex-pcode/amphtml,jridgewell/amphtml,donttrustthisbot/amphtml,aghassemi/amphtml,jmadler/amphtml,laterpay/amphtml,BenDz/amphtml,powdercloud/amphtml,clawr/amphtml,ampproject/amphtml,widespace-os/amphtml,bpaduch/amphtml,nexxtv/amphtml,taboola/amphtml,kamtschatka/amphtml,rsimha-amp/amphtml,bpaduch/amphtml,sebastianbenz/amphtml,ampproject/amphtml,media-net/amphtml,honeybadgerdontcare/amphtml,pedjoni8/amphtml,media-net/amphtml,luciancrasovan/amphtml,clawr/amphtml,Gregable/amphtml,BenDz/amphtml,alanorozco/amphtml,chaveznvg/amphtml,alanorozco/amphtml,kamtschatka/amphtml,mhc-gh/amphtml,alanorozco/amphtml,techhtml/amphtml,cvializ/amphtml,bpaduch/amphtml,widespace-os/amphtml,jpettitt/amphtml,zhouyx/amphtml,jmadler/amphtml,danielrozenberg/amphtml,Gregable/amphtml,widespace-os/amphtml,jmadler/amphtml,nexxtv/amphtml,laterpay/amphtml,yandex-pcode/amphtml,clawr/amphtml,adup-tech/amphtml,kamtschatka/amphtml,chaveznvg/amphtml,BenDz/amphtml,camelburrito/amphtml,Gregable/amphtml,shawnbuso/amphtml,mhc-gh/amphtml,dotandads/amphtml,Gregable/amphtml,comScore/amphtml,honeybadgerdontcare/amphtml,BenDz/amphtml,media-net/amphtml,prateekbh/amphtml,adup-tech/amphtml,ampproject/amphtml,cvializ/amphtml,dotandads/amphtml,prateekbh/amphtml,danielrozenberg/amphtml,smartadserver/amphtml,media-net/amphtml,zhangsu/amphtml,danielrozenberg/amphtml,cory-work/amphtml,yandex-pcode/amphtml,prateekbh/amphtml,camelburrito/amphtml,donttrustthisbot/amphtml,nexxtv/amphtml,jridgewell/amphtml,smartadserver/amphtml,pedjoni8/amphtml,yieldbot/amphtml,erwinmombay/amphtml,shawnbuso/amphtml,jpettitt/amphtml,honeybadgerdontcare/amphtml,sriramkrish85/amphtml,comScore/amphtml,luciancrasovan/amphtml,yieldmo/amphtml,mhc-gh/amphtml,widespace-os/amphtml,aghassemi/amphtml,shawnbuso/amphtml,taboola/amphtml,src-code/amphtml,zhangsu/amphtml,koshka/amphtml,engtat/amphtml,aghassemi/amphtml,sriramkrish85/amphtml,comScore/amphtml,dotandads/amphtml,aghassemi/amphtml,powdercloud/amphtml,koshka/amphtml,nexxtv/amphtml,comScore/amphtml,bpaduch/amphtml,taboola/amphtml,nexxtv/amphtml,camelburrito/amphtml,pedjoni8/amphtml,sriramkrish85/amphtml,cory-work/amphtml,yieldbot/amphtml,voyagegroup/amphtml,erwinmombay/amphtml,techhtml/amphtml,shawnbuso/amphtml,pedjoni8/amphtml,mhc-gh/amphtml,camelburrito/amphtml,yieldbot/amphtml,bpaduch/amphtml,clawr/amphtml,zhangsu/amphtml,jpettitt/amphtml,powdercloud/amphtml,shawnbuso/amphtml,taboola/amphtml,pedjoni8/amphtml,alanorozco/amphtml,adup-tech/amphtml,rsimha-amp/amphtml,shawnbuso/amphtml,cvializ/amphtml,lannka/amphtml,camelburrito/amphtml,bpaduch/amphtml,lannka/amphtml,donttrustthisbot/amphtml,media-net/amphtml,powdercloud/amphtml,nexxtv/amphtml,erwinmombay/amphtml,laterpay/amphtml,tlong2/amphtml,donttrustthisbot/amphtml,laterpay/amphtml,jmadler/amphtml,ampproject/amphtml,clawr/amphtml,honeybadgerdontcare/amphtml,mhc-gh/amphtml,tlong2/amphtml,chaveznvg/amphtml,aghassemi/amphtml,sebastianbenz/amphtml,sharethrough/amphtml,dotandads/amphtml,sharethrough/amphtml,cory-work/amphtml,voyagegroup/amphtml,smartadserver/amphtml,mhc-gh/amphtml,chaveznvg/amphtml,taboola/amphtml,Gregable/amphtml,src-code/amphtml,powdercloud/amphtml,ampproject/amphtml,voyagegroup/amphtml,lannka/amphtml,clawr/amphtml,yieldbot/amphtml,media-net/amphtml,cvializ/amphtml,rsimha-amp/amphtml,ampproject/amphtml,engtat/amphtml,donttrustthisbot/amphtml,ampproject/amphtml,danielrozenberg/amphtml,koshka/amphtml,adup-tech/amphtml,yieldmo/amphtml,danielrozenberg/amphtml,techhtml/amphtml,voyagegroup/amphtml,lannka/amphtml,erwinmombay/amphtml,yandex-pcode/amphtml,src-code/amphtml,koshka/amphtml,Gregable/amphtml,src-code/amphtml,dotandads/amphtml,pedjoni8/amphtml,lannka/amphtml,luciancrasovan/amphtml,danielrozenberg/amphtml,zhouyx/amphtml,sebastianbenz/amphtml,donttrustthisbot/amphtml,comScore/amphtml,yieldbot/amphtml,sebastianbenz/amphtml,sebastianbenz/amphtml,rsimha-amp/amphtml,camelburrito/amphtml,engtat/amphtml,engtat/amphtml,yandex-pcode/amphtml,smartadserver/amphtml,sharethrough/amphtml,yandex-pcode/amphtml,mhc-gh/amphtml,jridgewell/amphtml,voyagegroup/amphtml,kamtschatka/amphtml,techhtml/amphtml,luciancrasovan/amphtml,sharethrough/amphtml,sharethrough/amphtml,adup-tech/amphtml,alanorozco/amphtml,jridgewell/amphtml,cory-work/amphtml,rsimha-amp/amphtml,taboola/amphtml,sriramkrish85/amphtml,jpettitt/amphtml,engtat/amphtml,danielrozenberg/amphtml,cory-work/amphtml,erwinmombay/amphtml,comScore/amphtml,widespace-os/amphtml,prateekbh/amphtml,powdercloud/amphtml,yandex-pcode/amphtml,tlong2/amphtml,media-net/amphtml,voyagegroup/amphtml