Fix EZP-22910: Formtoken exception is thrown when logging in without debug redirection

https://jira.ez.no/browse/EZP-22910

Regression from [EZP-22767](https://jira.ez.no/browse/EZP-22767).

`LegacyHelper` is filled even if the result is an `ezpKernelRedirect`, triggering a form token exception when request is POST. Makes sense since `LegacyHelper` uses `runCallback()` which in this case re-triggers request parsing by the legacy kernel. As request is POST, it legitimately looks for the form token.

For some reason, the issue does not happen with XDebug on.