[metadata] Skip null vtable entires when checking covariant overrides

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

However if one of the ancestor classes is abstract, it could have an
abstract override for hte 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 is abstract).  In such cases we can skip over the class
and continue with its parent.

Fixes
#76312

src/mono/mono/metadata/class-setup-vtable.c

@@ -1656,6 +1656,10 @@ check_vtable_covariant_override_impls (MonoClass *klass, MonoMethod **vtable, in
 				break;
 			MonoMethod *prev_impl = cur_class->vtable[slot];
 
+			// if the current class re-abstracted the method, it may not be there.
+			if (!prev_impl)
+				continue;
+
 			if (prev_impl != last_checked_prev_override) {
 				/*
 				 * the new impl should be subsumed by the prior one, ie this