sync: fix manifest sync-j handling

Since --jobs defaults to 0, not None, we never pull the value out
of the manifest.  Treat values of 0 and None the same to fix.

Bug: http://b/239712300
Bug: http://b/260908907
Change-Id: I9b1026682072366616825fd72f90bd90c10a252f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/354254


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Sam Saccone <samccone@google.com>