Use null rather than an empty string when not rendering filter
Use null rather than an empty string when not rendering filter

Fixes #133

cklab/reactable,conversocial/reactable,glittershark/reactable,KendallPark/reactable,thebinarium/reactable,JonathanBruce/reactable,JonathanBruce/reactable,jmarceli/reactable,cklab/reactable,Widen/reactable,thebinarium/reactable,cythrawll/reactable,reggi/reactable,jmarceli/reactable,StefanYohansson/reactable,Widen/reactable,clarkie/reactable,glittershark/reactable,mobinni/reactable,conversocial/reactable,Theadd/reactable,davidblurton/reactable,nic/reactable,nic/reactable,cythrawll/reactable,Theadd/reactable,clarkie/reactable,hu9o/reactable,lovewitty/reactable,davidblurton/reactable,devZdev/reactable,wemcdonald/reactable,StefanYohansson/reactable,amplii/reactable,mobinni/reactable,lovewitty/reactable,jadamgreen1010/reactable,KendallPark/reactable,wemcdonald/reactable,reggi/reactable,amplii/reactable,devZdev/reactable,hu9o/reactable,jadamgreen1010/reactable