[DATABASE] Disable 'NULL' strings evaluation as SQL NULLs

Use $object->sqlValue('NULL') (identical to DataObject_Cast'ing) instead and
fix related issues like (email|sms)settings considering these NULLs as a
false positive for the E-Mail address still being set when it's been removed.

There could also be security implications to the now-disabled approach of
considering 'NULL' strings as SQL NULLs.

extlib/DB/DataObject.php

@@ -4168,8 +4168,7 @@ class DB_DataObject
             $method = $value;
             $value = func_get_arg(1);
         }
-        //require_once 'DB/DataObject/Cast.php';
-        require_once 'Cast.php';
+        require_once 'DB/DataObject/Cast.php';
         return call_user_func(array('DB_DataObject_Cast', $method), $value);
     }