[bot_update] Also bootstrap in the case that there are 0 pack files

This can happen if the cache repo was init'd, but has no pack files;
previously it would try to fetch from scratch.

R=ehmaldonado@chromium.org, tandrii@chromium.org

No-Presubmit: true
Change-Id: I97e863dd6c2ecfd00fdb4238dda48e0d3929c4f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1941337
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>

git_cache.py

@@ -486,11 +486,12 @@ class Mirror(object):
     pack_files = []
     if os.path.isdir(pack_dir):
       pack_files = [f for f in os.listdir(pack_dir) if f.endswith('.pack')]
-      self.print('%s has %d .pack files, re-bootstrapping if >%d' %
+      self.print('%s has %d .pack files, re-bootstrapping if ==0 or >%d' %
                 (self.mirror_path, len(pack_files), GC_AUTOPACKLIMIT))
 
     should_bootstrap = (force or
                         not self.exists() or
+                        len(pack_files) == 0 or
                         len(pack_files) > GC_AUTOPACKLIMIT)
 
     if not should_bootstrap: