Fall back to non-localized string if no translation is available

Summary:
We've seen a couple of `EXC_BAD_ACCESS` crashes in [`RCTParagraphComponentAccessibilityProvider.mm:L125`](https://github.com/facebook/react-native/blob/52d8a797e7a6be3fa472f323ceca4814a28ef596/React/Fabric/Mounting/ComponentViews/Text/RCTParagraphComponentAccessibilityProvider.mm#L125), most likely explained by [RCTLocalizationProvider RCTLocalizedString] returning nil, which will happen in the case that a language pack has no entry for the input string.

This is a small defensive change to fall back to the input if there's no better alternative.

Changelog:
[iOS][Fixed] - `RCTLocalizationProvider` Fall back to input when no localization is available

Reviewed By: luluonet

Differential Revision: D35583786

fbshipit-source-id: e8ae6ff61518e105301e7e51f5d8f43290fb20bf

React/Fabric/RCTLocalizationProvider.mm

@@ -26,15 +26,21 @@ @implementation RCTLocalizationProvider
 
 + (NSString *)RCTLocalizedString:(NSString *)oldString withDescription:(NSString *)description
 {
+  NSString *candidate = nil;
+
   if (_delegate != nil) {
-    return [_delegate localizedString:oldString withDescription:description];
+    candidate = [_delegate localizedString:oldString withDescription:description];
+  }
+
+  if (candidate == nil && _languagePack != nil) {
+    candidate = _languagePack[oldString];
   }
 
-  if (_languagePack != nil) {
-    return _languagePack[oldString];
+  if (candidate == nil) {
+    candidate = oldString;
   }
 
-  return oldString;
+  return candidate;
 }
 
 @end