bug #16897 [Form] Fix constraints could be null if not set (DZunke)

This PR was squashed before being merged into the 2.3 branch (closes #16897).

Discussion
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[Form] Fix constraints could be null if not set

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

If the form options has no key for constraints the default should be an empty array to be ignored by the loop in the FormValidator. The default without this fix is ```null``` and foreach will throw an error.

The "Bug" also still exists in master-Branch.

Commits
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f80e0eb [Form] Fix constraints could be null if not set

Author: fabpot

Extension/Validator/Constraints/FormValidator.php

@@ -45,7 +45,7 @@ public function validate($form, Constraint $constraint)
 
             // Validate the data against the constraints defined
             // in the form
-            $constraints = $config->getOption('constraints');
+            $constraints = $config->getOption('constraints', array());
             foreach ($constraints as $constraint) {
                 foreach ($groups as $group) {
                     if (in_array($group, $constraint->groups)) {

Tests/Extension/Validator/Constraints/FormValidatorTest.php

@@ -123,6 +123,17 @@ public function testDontValidateIfParentWithoutCascadeValidation()
         $this->assertNoViolation();
     }
 
+    public function testNotExistingConstraintIndex()
+    {
+        $object = new \stdClass();
+        $form = new FormBuilder('name', '\stdClass', $this->dispatcher, $this->factory);
+        $form = $form->setData($object)->getForm();
+
+        $this->validator->validate($form, new Form());
+
+        $this->assertNoViolation();
+    }
+
     public function testValidateConstraintsEvenIfNoCascadeValidation()
     {
         $object = $this->getMock('\stdClass');