RN: Fix NSInvalidArgumentException for Invalid Font Family Name

Summary:
We observed that in certain production scenarios, `[UIFont fontNamesForFamilyName:familyName]` returns `nil`.

This is problematic because we cannot insert `nil` (which is not an object type) into `NSCache`. Currently, this is causing `NSInvalidArgumentException` to be thrown.

This fix works around the problem by guarding against `nil`.

Changelog:
[iOS][Fixed] - Fix NSInvalidArgumentException for invalid font family names.

Reviewed By: fkgozali

Differential Revision: D31011394

fbshipit-source-id: a9eb9ce69efd832acca65787c665fcbb7b42a795

Author: yungsters

React/Views/RCTFont.mm

@@ -172,7 +172,7 @@ struct __attribute__((__packed__)) CacheKey {
 
   auto names = [cache objectForKey:familyName];
   if (!names) {
-    names = [UIFont fontNamesForFamilyName:familyName];
+    names = [UIFont fontNamesForFamilyName:familyName] ?: [NSArray new];
     [cache setObject:names forKey:familyName];
   }
   return names;