Fixes a crash when NSLocale returns nil for languageCode or countryCode
properties. This can happen when the app launches without locale
environment variables set.
The crash occurs at `src/os/locale.zig:87-88` when trying to call
`getProperty()` on a nil object. The fix adds a null check and falls
back to `en_US.UTF-8` instead of dereferencing null.
## Testing
Tested by running with locale variables unset:
```bash
unset LC_ALL && ./zig-out/Ghostty.app/Contents/MacOS/ghostty
```
Before: segmentation fault
After: launches successfully with fallback locale
3548acfac6