Fixes default value for back button when title is empty (#6552)
Fixes default value for back button when title is empty (#6552)

When the current navigation stack presents a new view controller with the topbar included, if the previous view controller didn't had a `{title: {text: string}}` defined, the app crashed. 

This commit fixes this issue which, by default, should render the back button with no text. 

Co-authored-by: Guy Carmeli <8c9c49408bd3a4a9650639faef73211de48a34a2@users.noreply.github.com>
3sidedcube/react-native-navigation,3sidedcube/react-native-navigation,wix/react-native-navigation,3sidedcube/react-native-navigation,wix/react-native-navigation,wix/react-native-navigation,wix/react-native-navigation,wix/react-native-navigation,3sidedcube/react-native-navigation,wix/react-native-navigation