Fix PropertyTag::setPropertyName() deprecation warning

Signed-off-by: Serg Lifinsky <[email protected]>

Author: lifinsky

Diff for: src/Generator/DocBlock/Tag/PropertyTag.php

@@ -37,7 +37,7 @@ public function getName()
      */
     public function setPropertyName($propertyName)
     {
-        $this->propertyName = ltrim($propertyName, '$');
+        $this->propertyName = $propertyName ? ltrim($propertyName, '$') : null;
         return $this;
     }