Check for nulls
Check for nulls

some values of a style can be nulls and doesnt need a replacement. we need to skeep them.
Plese not that in the cases like 'direction' (#L44) we want replacment for null anyway.
So only when we invoke function we check for null.

Fix for this issue:
https://github.com/callemall/material-ui/issues/4364
verdan/material-ui,kasra-co/material-ui,pancho111203/material-ui,mit-cml/iot-website-source,mtsandeep/material-ui,ichiohta/material-ui,frnk94/material-ui,w01fgang/material-ui,hwo411/material-ui,verdan/material-ui,pancho111203/material-ui,hai-cea/material-ui,ichiohta/material-ui,janmarsicek/material-ui,kittyjumbalaya/material-components-web,owencm/material-ui,hai-cea/material-ui,igorbt/material-ui,matthewoates/material-ui,andrejunges/material-ui,w01fgang/material-ui,mmrtnz/material-ui,mmrtnz/material-ui,lawrence-yu/material-ui,andrejunges/material-ui,ruifortes/material-ui,igorbt/material-ui,pomerantsev/material-ui,manchesergit/material-ui,janmarsicek/material-ui,frnk94/material-ui,lawrence-yu/material-ui,kittyjumbalaya/material-components-web,pomerantsev/material-ui,hwo411/material-ui,kasra-co/material-ui,manchesergit/material-ui,janmarsicek/material-ui,mit-cml/iot-website-source,mit-cml/iot-website-source,janmarsicek/material-ui,ArcanisCz/material-ui,ruifortes/material-ui,ArcanisCz/material-ui,owencm/material-ui,matthewoates/material-ui,mtsandeep/material-ui