Summary:
checking for background == null to fix this error:
```java.lang.NullPointerException: Attempt to invoke virtual method 'android.graphics.drawable.Drawable$ConstantState android.graphics.drawable.Drawable.getConstantState()' on a null object reference
at com.facebook.react.views.textinput.ReactTextInputManager.setUnderlineColor(:4)
```
Changelog:
[Internal][Fixed] - check for null background in Android TextInput setUnderlineColor
Reviewed By: beatthat
Differential Revision: D40451369
fbshipit-source-id: 8618d5ff73f517f714c26a0b17af078493822244
Sharon Zheng
b51a350c1e