fix(iOS): restore native behavior of auto shortening back button title (

software-mansion#2105)

## Description

Restore the iOS native behavior of automatically shorting the title of
the header back button to "Back" if there is not enough space, which is
documented
[here](https://reactnavigation.org/docs/header-buttons#customizing-the-back-button)[^1]
but does not behave as expected since v3.21.

Fixes software-mansion#1589.


## Changes

* Assign to `backBarButtonItem` only if actual customizations of the
back button are being made.


## Screenshots / GIFs

| Before | After |
|--------|------|
| ![Broken
1](https://github.com/software-mansion/react-native-screens/assets/3784687/880eaecb-54d9-48d3-95bd-5f8e6cd7b066)
| ![Working
1](https://github.com/software-mansion/react-native-screens/assets/3784687/201e8006-544d-43ee-95e3-308e2f926566)
|


## Test code and steps to reproduce

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## Checklist

- [ ] Included code example that can be used to test this change
- [ ] Updated TS types
- [ ] Updated documentation: <!-- For adding new props to native-stack
-->
- [ ]
https://github.com/software-mansion/react-native-screens/blob/main/guides/GUIDE_FOR_LIBRARY_AUTHORS.md
- [ ]
https://github.com/software-mansion/react-native-screens/blob/main/native-stack/README.md
- [ ]
https://github.com/software-mansion/react-native-screens/blob/main/src/types.tsx
- [ ]
https://github.com/software-mansion/react-native-screens/blob/main/src/native-stack/types.tsx
- [ ] Ensured that CI passes

[^1]: According to the document, 'On iOS this includes a label next to
the button, which shows the title of the previous screen when the title
fits in the available space, otherwise it says "Back".'

---------

Co-authored-by: Kacper Kafara <kacperkafara@gmail.com>

ios/RNSScreenStackHeaderConfig.mm

@@ -511,8 +511,17 @@ + (void)updateViewController:(UIViewController *)vc
                                                                              action:nil];
   [backBarButtonItem setMenuHidden:config.disableBackButtonMenu];
 
+  auto isBackButtonCustomized = !isBackTitleBlank || config.disableBackButtonMenu;
+
   if (config.isBackTitleVisible) {
-    if (config.backTitleFontFamily || config.backTitleFontSize) {
+    if ((config.backTitleFontFamily &&
+         // While being used by react-navigation, the `backTitleFontFamily` will
+         // be set to "System" by default - which is the system default font.
+         // To avoid always considering the font as customized, we need to have an additional check.
+         // See: https://github.com/software-mansion/react-native-screens/pull/2105#discussion_r1565222738
+         ![config.backTitleFontFamily isEqual:@"System"]) ||
+        config.backTitleFontSize) {
+      isBackButtonCustomized = YES;
       NSMutableDictionary *attrs = [NSMutableDictionary new];
       NSNumber *size = config.backTitleFontSize ?: @17;
       if (config.backTitleFontFamily) {
@@ -535,9 +544,17 @@ + (void)updateViewController:(UIViewController *)vc
     // When backBarButtonItem's title is null, back menu will use value
     // of backButtonTitle
     [backBarButtonItem setTitle:nil];
+    isBackButtonCustomized = YES;
     prevItem.backButtonTitle = resolvedBackTitle;
   }
-  prevItem.backBarButtonItem = backBarButtonItem;
+
+  // Prevent unnecessary assignment of backBarButtonItem if it is not customized,
+  // as assigning one will override the native behavior of automatically shortening
+  // the title to "Back" or hide the back title if there's not enough space.
+  // See: https://github.com/software-mansion/react-native-screens/issues/1589
+  if (isBackButtonCustomized) {
+    prevItem.backBarButtonItem = backBarButtonItem;
+  }
 
   if (@available(iOS 11.0, *)) {
     if (config.largeTitle) {