Promote object validity checks to release builds

Extra:
- Optimized the debug-only check about why the object is null to determine if it's because it has been deleted (the RC is enough; no need to check the ObjectDB).
- Because of the previous point. the debugger being attached is not required anymore for giving the "Object was deleted" error; from now, it only matters that it's a debug build.
- `is_instance_valid()` is now trustworthy. It will return `true` if, and only if, the last object assigned to a `Variant` is still alive (and not if a new object happened to be created at the same memory address of the old one).
- Replacements of `instance_validate()` are used where possible `Variant::is_invalid_object()` is introduced to help with that. (GDScript's `is_instance_valid()` is good.)

scene/debugger/script_debugger_remote.cpp

@@ -101,7 +101,7 @@ void ScriptDebuggerRemote::_put_variable(const String &p_name, const Variant &p_
 	packet_peer_stream->put_var(p_name);
 
 	Variant var = p_variable;
-	if (p_variable.get_type() == Variant::OBJECT && !ObjectDB::instance_validate(p_variable)) {
+	if (p_variable.get_type() == Variant::OBJECT && p_variable.operator Object *() == nullptr) {
 		var = Variant();
 	}