Fix #17019 : overrides with unknown base setting

If at the time of the _GLOBAL_DEF call a setting itself was unknown,
the function would always return the supplied default value instead of
checking for overrides. This commit changes that, lookup now always
happens which correctly takes overrides into account.

core/project_settings.cpp

@@ -816,12 +816,11 @@ Error ProjectSettings::save_custom(const String &p_path, const CustomMap &p_cust
 Variant _GLOBAL_DEF(const String &p_var, const Variant &p_default) {
 
 	Variant ret;
-	if (ProjectSettings::get_singleton()->has_setting(p_var)) {
-		ret = ProjectSettings::get_singleton()->get(p_var);
-	} else {
+	if (!ProjectSettings::get_singleton()->has_setting(p_var)) {
 		ProjectSettings::get_singleton()->set(p_var, p_default);
-		ret = p_default;
 	}
+	ret = ProjectSettings::get_singleton()->get(p_var);
+
 	ProjectSettings::get_singleton()->set_initial_value(p_var, p_default);
 	ProjectSettings::get_singleton()->set_builtin_order(p_var);
 	return ret;