siso: reapi.UpdateProjectID returns original projectID if not RBE

In android soong, siso runs in nsjail and reapi address is
not RBE, but unix domain socket to siso proxy.
In this case reapi.UpdateProjectID returned empty,
so it would disable cloud profiler etc.

Just return original projectID (given by -project flag)
when reapi address it not RBE.

Change-Id: Ie7b46e8a9eaab798cce1e98a8eee4c9dbd513909
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8192135
Reviewed-by: Richard Wang <richardwa@google.com>
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>