Add missing nullptr check

In theory, having a sessionless websocket isn't possible.  In practice,
this did come up when an ownership issue caused UB, which is how I saw
this.

Tested:
Tested with scripts/websocket_test.py and saw sensor values streaming by
as expected.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I7cc9c9660c8207ba857e6f6f14f010eaf79b73ef