[Form] removed a constraint in PropertyPath as the path can definitel…

…y be an empty string for errors attached on the main form (when using a constraint defined with the 'validation_constraint' option)

Util/PropertyPath.php

@@ -55,7 +55,7 @@ class PropertyPath implements \IteratorAggregate
      */
     public function __construct($propertyPath)
     {
-        if ('' === $propertyPath || null === $propertyPath) {
+        if (null === $propertyPath) {
             throw new InvalidPropertyPathException('The property path must not be empty');
         }