prevent error `RawText "" must be wrapped in an explicit <Text> compo… (#2415)

this react-native error has no adequate stack, and i waste 4 hours for juggling pieces, until i get the idea to trace error by myself and finded this bug

packages/react-navigation/src/views/Header/HeaderBackButton.js

@@ -91,7 +91,7 @@ class HeaderBackButton extends React.PureComponent<Props, State> {
             source={asset}
           />
           {Platform.OS === 'ios' &&
-            title && (
+            typeof backButtonTitle === 'string' && (
               <Text
                 onLayout={this._onTextLayout}
                 style={[