Update QueryGenerator.php

Does Not Contain (operator k) must allow for NULL values as well as NOT LIKE values (e.g. in multipicklist).
FIXES error as stated at: vtiger/vtigercrm#1682

See merge request !849

fix login in migration page

In the migration page, there is an authentication check to allow user to login. However, the `Migration_Extract_Action` requires login by default.

Since the check was done before authentication check, this causes an authentication failure whether the username and password is correct or not.

Making login not required by `Migration_Extract_Action` will allow the authentication.

This commit also fixes the login issue for `Migration_Extract_Action` since there are something missing in the `$_SESSION`

See merge request !854