[metadata] Skip null vtable entries when checking covariant return overrides

[lambdageek]

When there are covariant return overrides, we check the vtable slot in every parent class for signature compatibility with the proposed override.

However if one of the ancestor classes is abstract, it could have an abstract override for the method:

    class Base {
      public virtual Base Method() => this;
    }
    public abstract Intermediate : Base {
      public override abstract Base Method();
    }
    public Leaf : Intermediate {
      public override Leaf Method() => this;
    }

In this case when we're checking that Leaf.Method is compatible with the vtable slot in Intermediate, we will see a null method (since Intermediate.Method is abstract). In such cases we can skip over the class and continue with its parent.

Fixes
#76312

[lambdageek]

I think we should backport this for net7 and net6

[lambdageek]

/azp run runtime-extra-platforms

[lambdageek]

/backport to release/7.0

[github-actions]

Started backporting to release/7.0: https://github.com/dotnet/runtime/actions/runs/3146190259

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[lambdageek]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/runtime/actions/runs/3146255970

[lambdageek]

runtime-extra-platforms (Build tvOS arm64 Release AllSubsets_Mono) is failing repeatedly while trying to set up a tunnel to the device:

[10:39:13] dbug: [TCP tunnel] Xamarin.Hosting: Attempting USB tunnel between the port 61033 on the device and the port 61033 (27118) on the mac: 61

I think it's an infra problem, not my PR...