Merge branch '4.4' into 5.2

* 4.4:
  [Serializer][Validator] Update some phpDoc relative to "getters"
  Update README.md
  [SecurityBundle] Empty line starting with dash under "access_control" causes all rules to be skipped
  [Cache] Apply NullAdapter as Null Object

Author: derrabus

Mapping/ClassMetadata.php

@@ -264,7 +264,7 @@ public function addPropertyConstraints(string $property, array $constraints)
      * Adds a constraint to the getter of the given property.
      *
      * The name of the getter is assumed to be the name of the property with an
-     * uppercased first letter and either the prefix "get" or "is".
+     * uppercased first letter and the prefix "get", "is" or "has".
      *
      * @return $this
      */

Mapping/GetterMetadata.php

@@ -18,7 +18,7 @@
  * method.
  *
  * A property getter is any method that is equal to the property's name,
- * prefixed with either "get" or "is". That method will be used to access the
+ * prefixed with "get", "is" or "has". That method will be used to access the
  * property's value.
  *
  * The getter will be invoked by reflection, so the access of private and