[#52] Fix building leaderboard error with no battles (#53)

This change fixes an error that occurs when the leaderboard is built with no battles.

Co-authored-by: suhyun.kang <suhyun.kang@yanolja.group>

.gitignore

@@ -1,4 +1,4 @@
 venv
 *.log
 __pycache__
-credentials.json
+credentials*.json

leaderboard.py

@@ -64,6 +64,9 @@ def load_elo_ratings(tab):
         "winner": data["winner"]
     })
 
+  if not battles:
+    return
+
   battles = pd.DataFrame(battles)
   ratings = compute_elo(battles)