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

Since commit 454fdaf (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>

subcmds/sync.py

@@ -901,7 +901,7 @@ def _ProcessResults(pool, pm, results_sets):
                 objdir_project_map.setdefault(project.objdir, []).append(index)
             projects_list = list(objdir_project_map.values())
 
-            jobs = min(opt.jobs_network, len(projects_list))
+            jobs = max(1, min(opt.jobs_network, len(projects_list)))
 
             # We pass the ssh proxy settings via the class.  This allows
             # multiprocessing to pickle it up when spawning children.  We can't