[expo-localization] Fix constants exporting

[lukmccall]

Why

Resolve #5735.

How

Wrapped region variable into UMNullIfNil macro, because [[NSLocale currentLocale] objectForKey:NSLocaleCountryCode] could return nil on iOS simulator.

[sjchmiela]

If I'm correct, if this would be the cause of the issue linked, reporting users would get crashes (could not insert nil to a dictionary) instead of JS problems. Are you sure this fixes the issue? 🤔

[lukmccall]

That's true, you can't insert nil to a dictionary.
However, this code compiles and doesn't throw any exception:

NSString *b = nil;
return @{
  @"a": b
}

Unfortunately, bridge can't read correct values from this dictionary.

[brentvatne]

we should add a test that would have caught this regression to ensure it doesn't happen again

[marudy]

I still get the same issue after upgrading expo localisation to current latest^8.0.0",.

Do we have an open issue about it and maybe an upcoming fix? Or am I missing something? Thanks