sync: Do not fail to sync a manifest with no projects

Since commit 454fdaf1 (v2.48), syncing a
manifest without any projects would result in:

  Repo command failed: RepoUnhandledExceptionError
          Number of processes must be at least 1

Bug: 377546300
Change-Id: Iaa2f6a3ac64542ad65a19c0eef449f53c09cae67
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/443442


Reviewed-by: Erik Elmeke <erik@haleytek.corp-partner.google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Tested-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>