[Form] fix return type declarations

Author: nicolas-grekas

PropertyPathBuilder.php

@@ -193,7 +193,7 @@ public function getLength()
     /**
      * Returns the current property path.
      *
-     * @return PropertyPathInterface The constructed property path
+     * @return PropertyPathInterface|null The constructed property path
      */
     public function getPropertyPath()
     {

PropertyPathInterface.php

@@ -40,7 +40,7 @@ public function getLength();
      *
      * If this property path only contains one item, null is returned.
      *
-     * @return PropertyPath|null The parent path or null
+     * @return self|null The parent path or null
      */
     public function getParent();