Fix EZP-23683: session is always started for legacy_mode SiteAccess

JIRA: https://jira.ez.no/browse/EZP-23683

Problem: a session is always started for siteaccesses in legacy_mode.

It seems `Symfony\Component\HttpFoundation\Session::has( $name ) ` actually starts a session if one does not yet exist, so this PR adds an additional check to see if one is already started.

Tests: manual.