[macos] early return to suppress clang-tidy warning (flutter/engine#56588)

If we disable the NSAssert during release mode, this code will continue after NSAssert failure, which will then pass a `nil` value into a dictionary (which cannot have a nil value), hence the clang-tidy warning here flutter/engine#53005 (comment)

*List which issues are fixed by this PR. You must list at least one issue.*
flutter#148279
flutter#157837

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: hellohuanlin

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponder.mm

@@ -131,10 +131,13 @@ - (void)handleEvent:(NSEvent*)event callback:(FlutterAsyncKeyCallback)callback {
         return;
       }
       break;
-    default: {
-      NSAssert(false, @"Unexpected key event type (got %lu).", event.type);
-      callback(false);
-    }
+    default:
+      [[unlikely]] {
+        NSAssert(false, @"Unexpected key event type (got %lu).", event.type);
+        callback(false);
+        // This should not happen. Return to suppress clang-tidy warning on `type` being nil.
+        return;
+      }
   }
   _previouslyPressedFlags = modifierFlags;
   NSMutableDictionary* keyMessage = [@{