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