fix(sessions): guard None event.actions before reading state_delta

BaseSessionService guards `event.actions` for None before reading
state_delta in _apply_temp_state, _trim_temp_delta_state, and
_update_session_state, matching prior behavior for callers that
build events with actions unset. InMemorySessionService applies the
same guard in append_event.

Change-Id: I6598ac68711ebb360f8081c2f895e1dc17942ad5