fix: auto fill serialize/deserialize for classes

[paulpach]

fixes #2117

This now works:

 class ArrayIntMessage : IMessageBase
    {
        public int[] array;

        // Mirror will fill out these empty methods
        public void Deserialize(NetworkReader reader) { }
        public void Serialize(NetworkWriter writer) { }
    }

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
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100.0% Coverage
0.0% Duplication

[paulpach]

Note this method is being called in 3 places:
one in MessageClassProcessor and 2 in NetworkBehaviorProcessor.

The NetworkBehaviorProcess is really a don't care, this condition cannot happen since it would not be invoked unless the class extends from NetworkBehavior. In this case, it will always resolve.
but even if it does not resolve, they handle the null case gracefully by not calling the base method.

MessageClassProcessor also calls this method, and if it is null it does not call the base method. That is precisely the behavior we want to resolve #2117

so all 3 places are perfectly fine receiving null.

The only way to trigger the error was #2117

I renamed the method to make it clearer that this may not find the method in the parent class.