[PHP] Fix parentSchema conditional causing parent call when parent isn't present (#8705)

* Fix parentSchema conditional

we cannot call `parent::` if parent isn't present. Everywhere else in the schema we're checking for `{{parentSchema}}` but here we're checking for `{{parent}}` which is causing errors when parent is not present

* Update samples

Author: buttilda

modules/openapi-generator/src/main/resources/php/model_generic.mustache

@@ -188,12 +188,12 @@ class {{classname}} {{#parentSchema}}extends {{{parent}}}{{/parentSchema}}{{^par
      */
     public function listInvalidProperties()
     {
-        {{#parent}}
+        {{#parentSchema}}
         $invalidProperties = parent::listInvalidProperties();
-        {{/parent}}
-        {{^parent}}
+        {{/parentSchema}}
+        {{^parentSchema}}
         $invalidProperties = [];
-        {{/parent}}
+        {{/parentSchema}}
 
         {{#vars}}
         {{#required}}

samples/client/petstore/php/OpenAPIClient-php/lib/Model/NullableClass.php

@@ -254,7 +254,7 @@ public function __construct(array $data = null)
      */
     public function listInvalidProperties()
     {
-        $invalidProperties = parent::listInvalidProperties();
+        $invalidProperties = [];
 
         return $invalidProperties;
     }