Make freed object different than null in comparison operators

This is so everything is consistent, as a freed object is not equivalent
to `null` in general. The booleanization of a freed object still returns
`false` to work as an easy check for validity of objects. Similarly, the
negation of a freed object returns `true`.

Also makes freed objects different from each other (if they are not the
same reference).

core/variant/variant.cpp

@@ -931,7 +931,7 @@ bool Variant::is_zero() const {
 			return *reinterpret_cast<const ::RID *>(_data._mem) == ::RID();
 		}
 		case OBJECT: {
-			return _get_obj().obj == nullptr;
+			return get_validated_object() == nullptr;
 		}
 		case CALLABLE: {
 			return reinterpret_cast<const Callable *>(_data._mem)->is_null();