Fix PHP8.1 sfForm - Unsupported operand types: array + null on sfForm::updateValues()

Avoid error :
Fatal error: Uncaught TypeError: Unsupported operand types: array + null in lib\form\sfForm.class.php:319

Step to reproduce :
$form = new sfForm();
$form->updateValues(array('foo' => 'value'));

PHP 5.3 coding style

May use sfForm::getValues(), but as sfForm::updateValues() is a hack, it could be used before bind ($this->isBound = true);

Author: Tybaze

lib/form/sfForm.class.php

@@ -316,7 +316,7 @@ public function isBound()
    */
   public function updateValues(array $values)
   {
-    $this->values = $values + $this->values;
+    $this->values = $values + (isset($this->values) ? $this->values : array());
 
     $this->updateValuesEmbeddedForms($values);
   }