fix(web): WASM - After a SignOut with an active authStateChange/idTokenChange a null check would be thrown

[MattyBoy4444]

Description

When WASM is being used, a SignOut with an active authStateChange/idTokenChange would throw a null check error

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[SelaseKay]

Hi @MattyBoy4444, thanks for the contribution! Could you also add a test covering this change? It would also be helpful to open an issue with reproducible steps and link this PR to it.

[MattyBoy4444]

Testing isn't setup in firebase_auth_web. What would you expect the test to do? Its just a simple null check error fix. Please merge.