fix: Handle null error message in promise rejection

Prevents crash when an exception with null message is caught and rejected
through the React Native bridge.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package/android/src/main/java/com/mrousavy/camera/utils/withPromise.kt

@@ -11,6 +11,6 @@ inline fun withPromise(promise: Promise, closure: () -> Any?) {
   } catch (e: Throwable) {
     e.printStackTrace()
     val error = if (e is CameraError) e else UnknownCameraError(e)
-    promise.reject("${error.domain}/${error.id}", error.message, error.cause)
+    promise.reject("${error.domain}/${error.id}", error.message ?: "Unknown error", error.cause)
   }
 }