bug #18072 [serializer][ObjectNormalizer] fixed Undefined attributeName. (aitboudad)

This PR was merged into the 3.1-dev branch.

Discussion
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[serializer][ObjectNormalizer] fixed Undefined attributeName.

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

Commits
-------

ee087a1 [serializer][ObjectNormalizer] fixed Undefined attributeName.

Author: fabpot

Normalizer/ObjectNormalizer.php

@@ -57,6 +57,7 @@ protected function extractAttributes($object, $format = null, array $context = a
             }
 
             $name = $reflMethod->name;
+            $attributeName = null;
 
             if (0 === strpos($name, 'get') || 0 === strpos($name, 'has')) {
                 // getters and hassers
@@ -66,7 +67,7 @@ protected function extractAttributes($object, $format = null, array $context = a
                 $attributeName = lcfirst(substr($name, 2));
             }
 
-            if ($this->isAllowedAttribute($object, $attributeName)) {
+            if (null !== $attributeName && $this->isAllowedAttribute($object, $attributeName)) {
                 $attributes[$attributeName] = true;
             }
         }