Fix Open Debugger dev menu item missing from iOS Bridgeless

Summary:
After a [recent change](https://github.com/facebook/react-native/commit/90296be1d4fab09a52e02dd09f34f819136d0a07) we break part of the integration with the debug menu, which is was using the presence/absence of the bridge to decide whether we were in bridge or bridgeless.

For backward compatibility reasosn, the bridge ivar is now populated with the bridgeProxy, so just checking whether is nil or not is not enough to verify whether we are in bridge or in bridgeless mode anymore.

## Changelog:
[iOS][Fixed] - Make sure that the Open Debugger appears in bridgeless mode

Reviewed By: fkgozali

Differential Revision: D56067897

fbshipit-source-id: e2501ed730ff35bc755c24ef400130c551032e28