[CodeQuality] Skip abstract class on CompleteDynamicPropertiesRector

[TomasVotruba]

Abstract class dynamic property might be accessed from a child class, so we cannot safely complete it. Skip abstract classes.

 abstract class SomeClass
 {
     public function set()
     {
         $this->value = 5;
     }
 }

Before: rule added public $value; to the abstract class. Now it is skipped, since a child class may declare/use that property.