RepositoryForm: Use null as default in methods add() and edit()

An empty array is now considered valid data in method
onUpdateRequest() which now allows to avoid unnecessary
data retrieval.

application/forms/RepositoryForm.php

@@ -149,7 +149,7 @@ public function shouldDelete()
      *
      * @return  $this
      */
-    public function add(array $data = array())
+    public function add(array $data = null)
     {
         $this->mode = static::MODE_INSERT;
         $this->data = $data;
@@ -164,7 +164,7 @@ public function add(array $data = array())
      *
      * @return  $this
      */
-    public function edit($name, array $data = array())
+    public function edit($name, array $data = null)
     {
         $this->mode = static::MODE_UPDATE;
         $this->identifier = $name;
@@ -239,7 +239,7 @@ protected function onInsertRequest()
     protected function onUpdateRequest()
     {
         $data = $this->getData();
-        if (empty($data)) {
+        if ($data === null) {
             $row = $this->repository
                 ->select()
                 ->from($this->getBaseTable())