Fix to top movers

Don't try processsing decks that don't have a score before the cut-off
date.

app/routers/stats.py

@@ -141,6 +141,7 @@ def top_movers(
             - score_tracker[deck_id]["start_score"],
         }
         for deck_id in score_tracker
+        if "start_score" in score_tracker[deck_id]
     ]
     return {
         "positive": nlargest(number_of_movers, calculateds, key=lambda x: x["diff"]),