Touchable: Restore underlayColor={null} Behavior

Summary:
The former implementations of `TouchableHighlight` used `defaultProps` for `underlayColor`. However, the newly landed implementations use `??` which falls back to the default behavior if the prop is `null`.

This restores the former behavior so that, for example, supplying `underlayColor={null}` to `TouchableHighlight` will not fallback to black. (It probably should always have, but the intention of my rewrite was not to introduce a breaking change.)

Changelog:
[General] [Fixed] - Restore behavior for `underlayColor={null}` in `TouchableHighlight`.

Reviewed By: zackargyle

Differential Revision: D18806494

fbshipit-source-id: 4d33810e2f754f980385d76d81dc0f34006f4337

Author: yungsters

Libraries/Components/Touchable/TouchableHighlight.js

@@ -237,7 +237,12 @@ class TouchableHighlight extends React.Component<Props, State> {
   _createExtraStyles(): ExtraStyles {
     return {
       child: {opacity: this.props.activeOpacity ?? 0.85},
-      underlay: {backgroundColor: this.props.underlayColor ?? 'black'},
+      underlay: {
+        backgroundColor:
+          this.props.underlayColor === undefined
+            ? 'black'
+            : this.props.underlayColor,
+      },
     };
   }