Make empty repo sync work

If the repo is empty, the repo pushing will be skipped, that means,
only empty repo will be created, and keep it empty like source repo.

.github/workflows/verify-on-ubuntu-org.yml

@@ -17,6 +17,6 @@ jobs:
         account_type: org
         # Only sync normal repo
         black_list: 'test'
-        white_list: 'normal,test'
+        white_list: 'normal,test,empty'
         force_update: true
-        debug: true
+        debug: true

hub-mirror/mirror.py

@@ -57,9 +57,19 @@ def create(self):
         print("(2/3) Creating...")
         self.hub.create_dst_repo(self.name)
 
+    def _check_empty(self, repo):
+        cmd = ["-n", "1", "--all"]
+        if repo.git.rev_list(*cmd):
+            return False
+        else:
+            return True
+
     @retry(wait=wait_exponential(), reraise=True, stop=stop_after_attempt(3))
     def push(self, force=False):
         local_repo = git.Repo(self.repo_path)
+        if self._check_empty(local_repo):
+            print("Empty repo %s, skip pushing." % self.src_url)
+            return
         cmd = ['set-head', 'origin', '-d']
         local_repo.git.remote(*cmd)
         try: