fix useless fallback at end of pairing

src/warchron/model/pairing.py

@@ -329,12 +329,13 @@ class Pairing:
         round: Round,
         remaining: List[str],
     ) -> None:
+        if not remaining:
+            return
         campaign = war.get_campaign_by_round(round.id)
         if campaign is None:
             raise DomainError("Campaign not found")
         match_counts = Pairing.build_match_count(campaign)
         available_battles = round.get_battles_with_places()
-        # FIXME no error when all participants allocated
         if not available_battles:
             raise DomainError("No available battle remaining")
         occupancy = {