do not create PR when no changes required

tools/sync_doc_repo.py

@@ -120,6 +120,10 @@ def process_repositories(args, service):
             dirs_exist_ok=True
         )
         repo_to.index.add([doc["rst_location"]])
+        if len(repo_to.index.diff("HEAD")) == 0:
+            # Nothing to commit
+            logging.debug("No changes.")
+            continue
         repo_to.index.commit(
             (
                 f"Synchronize {doc['title']}\n\n"