[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.

classes/Conversation.php

@@ -106,7 +106,7 @@ class Conversation extends Managed_DataObject
                 common_random_hexstr(8)
             );
             // locally generated Conversation objects don't get static URLs stored
-            $conv->url = DB_DataObject_Cast::sql('NULL');
+            $conv->url = $conv->sqlValue('NULL');
         }
         // This insert throws exceptions on failure
         $conv->insert();