login: fix two io.systemd.Login Varlink IDL issues (#40857)

Add missing NoSessionPIDFD error to the interface definition; the error
is emitted by vl_method_create_session() when the session leader process
does not have a pidfd available, but was never declared in the IDL.

Drop SD_JSON_MANDATORY from the ReleaseSession.Id dispatch entry so that
omitting the field (which the IDL allows via SD_VARLINK_NULLABLE)
resolves to the caller's own session via session_is_self(NULL).