[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.

modules/Favorite/FavoriteModule.php

@@ -75,7 +75,7 @@ class FavoriteModule extends ActivityVerbHandlerModule
             while ($user->fetch()) {
                 $user->setPref('email', 'notify_fave', $user->emailnotifyfav);
                 $orig = clone($user);
-                $user->emailnotifyfav = 'null';   // flag this preference as migrated
+                $user->emailnotifyfav = $user->sqlValue('NULL'); // flag this preference as migrated
                 $user->update($orig);
             }
             printfnq("DONE.\n");