Fix: sessiond: no rotation performed from null chunk to new chunk

The session daemon must rotate from a null trace chunk to a new trace
chunk when a session's current trace chunk is null. This situation
happens when a session is stopped, rotated, and started again.

This bug leaves existing streams in the "null" trace chunk and causes
the relay daemon to report a protocol error on the reception of the
first data packet following such a start.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>