_getText defaults to blank string when no default value set

Summary:
One of the potential fixes that grabbou suggested was to make _getText return an empty string in the worst case of null value and null defaultValue.
Closes https://github.com/facebook/react-native/pull/9553

Differential Revision: D3800913

fbshipit-source-id: 30d9c0a7384d39477a71947714eec3340ba5380f

Libraries/Components/TextInput/TextInput.js

@@ -575,7 +575,11 @@ const TextInput = React.createClass({
   _getText: function(): ?string {
     return typeof this.props.value === 'string' ?
       this.props.value :
-      this.props.defaultValue;
+      (
+        typeof this.props.defaultValue === 'string' ?
+        this.props.defaultValue :
+        ''
+      );
   },
 
   _setNativeRef: function(ref: any) {