[runtime] Fix Empty generic enumerator equality

On the CLR, two enumerators containing zero elements will
have reference equality if they are inflated with the same
generic type.

If comparing two enumerators which are not inflated (Array.Empty, for
instance), they are not equal when they contain zero elements.

This reference equality behavior is not compatible with treating
the enumerators as structs, as value types will not have reference
equality.