helper/schema: NewComputed values should be nil
helper/schema: NewComputed values should be nil

When working on this initially, I think I thought that since NewComputed
values in the diff were empty strings, that it was using the zero value.
After review, it doesn't seem like this is the case - so I have adjusted
NewComputed to pass nil values. There is also a guard now that keeps the
new value writer from accepting computed fields with non-nil values.

luis-silva/terraform,ewbankkit/terraform,HotelsDotCom/terraform,tpounds/terraform,youhong316/terraform,apparentlymart/terraform,ColinHebert/terraform,kjmkznr/terraform,rnaveiras/terraform,heimweh/terraform,ColinHebert/terraform,yanndegat/terraform,ColinHebert/terraform,ColinHebert/terraform,yanndegat/terraform,hooklift/terraform,gechr/terraform,antonbabenko/terraform,hashicorp/terraform,kjmkznr/terraform,justnom/terraform,drebes/terraform,VladRassokhin/terraform,asedge/terraform,grasshoppergroup/terraform,apparentlymart/terraform,mattray/terraform,fatmcgav/terraform,apparentlymart/terraform,pdecat/terraform,iljaskevic/terraform,kl4w/terraform,pradeepbhadani/terraform,hartzell/terraform,pryorda/terraform,yanndegat/terraform,HotelsDotCom/terraform,gechr/terraform,nlamirault/terraform,kl4w/terraform,heimweh/terraform,mattray/terraform,paultyng/terraform,chrisjharding/terraform,hartzell/terraform,vancluever/terraform,svanharmelen/terraform,brandontosch/terraform,kl4w/terraform,ddegoede/terraform,svanharmelen/terraform,fatmcgav/terraform,antonbabenko/terraform,brandontosch/terraform,vancluever/terraform,hooklift/terraform,gechr/terraform,youhong316/terraform,drebes/terraform,VladRassokhin/terraform,chrisjharding/terraform,mattray/terraform,tommynsong/terraform,grasshoppergroup/terraform,nlamirault/terraform,huydinhle/terraform,vancluever/terraform,iansheridan/terraform,rnaveiras/terraform,yanndegat/terraform,luis-silva/terraform,chrisjharding/terraform,VladRassokhin/terraform,huydinhle/terraform,youhong316/terraform,drebes/terraform,chrisjharding/terraform,iansheridan/terraform,lusis/terraform,paultyng/terraform,brandontosch/terraform,paultyng/terraform,grasshoppergroup/terraform,justnom/terraform,tommynsong/terraform,donaldguy/terraform,iljaskevic/terraform,heimweh/terraform,iansheridan/terraform,asedge/terraform,kjmkznr/terraform,ewbankkit/terraform,youhong316/terraform,ddegoede/terraform,HotelsDotCom/terraform,asedge/terraform,apparentlymart/terraform,pradeepbhadani/terraform,antonbabenko/terraform,tommynsong/terraform,hartzell/terraform,grasshoppergroup/terraform,brandontosch/terraform,hooklift/terraform,justnom/terraform,paultyng/terraform,apparentlymart/terraform,kl4w/terraform,pryorda/terraform,tpounds/terraform,VladRassokhin/terraform,iljaskevic/terraform,antonbabenko/terraform,iansheridan/terraform,HotelsDotCom/terraform,huydinhle/terraform,svanharmelen/terraform,ddegoede/terraform,mattray/terraform,pryorda/terraform,ddegoede/terraform,lusis/terraform,rnaveiras/terraform,tpounds/terraform,hashicorp/terraform,luis-silva/terraform,pradeepbhadani/terraform,lusis/terraform,pdecat/terraform,iljaskevic/terraform,huydinhle/terraform,fatmcgav/terraform,nlamirault/terraform,vancluever/terraform,antonbabenko/terraform,heimweh/terraform,pdecat/terraform,donaldguy/terraform,hashicorp/terraform,hartzell/terraform,pdecat/terraform,hooklift/terraform,donaldguy/terraform,svanharmelen/terraform,ddegoede/terraform,luis-silva/terraform,tpounds/terraform,ewbankkit/terraform,HotelsDotCom/terraform,asedge/terraform,lusis/terraform,justnom/terraform,fatmcgav/terraform,chrisjharding/terraform,heimweh/terraform,drebes/terraform,brandontosch/terraform,ewbankkit/terraform,donaldguy/terraform,kjmkznr/terraform