Prevent display the section title at startup.

This MR prevents the application to try to display the section title when no section is active. When the application is served at the root (no KRESUS_URL_PREFIX is set), this would lead to trying to translate client.menu.undefined, which generates a console error. When a KRESUS_URL_PREFIX is set, this would mean trying to translate client.menu.prefix where prefix is such KRESUS_URL_PREFIX === /prefix(/something).