Update AbstractPlatform.php

reverted @deeky666 suggestion. Since it failed on testBooleanNullConvertsToPHPValue test. 
with message Failed asserting that true is null.
Since I moved this code from BoolenType::convertToPHPValue in the first place, I presume the code there correct.

lib/Doctrine/DBAL/Platforms/AbstractPlatform.php

@@ -2592,7 +2592,7 @@ abstract class AbstractPlatform
      */
     public function convertFromBoolean($item)
     {
-        return null === $item ?: (bool) $item ;
+        return null === $item ? null: (bool) $item ;
     }
 
     /**