end_session() with end_tcsession='kill' could not terminate the session once new_session() was not finished.

[Kayla-zhang]

Hi,
When we are using new_session() and end_session() in stc_http.py, An issue was found.
Our process is trying to use new_session() to create session.
During the session creating in labserver (started, but not finished), our process was interrupted by TERM signal and it falls into graceful shutdown, thus end_session(end_tcsession='kill') is called, which is trying to terminate the session in labserver.
In the parameter specification of end_seesion method, it is "sid -- ID of session to end. None to use current session.".
However, end_session(end_tcsession='kill') is called to terminate current session, but failed. We could still see the session in labserver via session manager. It is because new_seesion() is not finshed and self._sid is None.
Please fix it to let end_session with end_tcsession='kill' work in this scenario. Thanks.