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 <guyca@users.noreply.github.com>

lib/ios/TopBarPresenter.m

@@ -136,7 +136,7 @@ - (void)setBackButtonOptions:(RNNBackButtonOptions *)backButtonOptions {
     [self setBackIndicatorImage:icon withColor:color];
 
     if (showTitle) {
-        backItem.title = title ? title : previousViewControllerInStack.navigationItem.title;
+        backItem.title = title ? title : (previousViewControllerInStack.navigationItem.title ? previousViewControllerInStack.navigationItem.title : @"");;
     } else {
         backItem.title = @"";
     }