fixup! adapt back with blocknote interface

Author: lunika

src/backend/core/api/viewsets.py

@@ -2328,4 +2328,6 @@ def reactions(self, request, *args, **kwargs):
         except models.Reaction.DoesNotExist as e:
             raise Http404() from e
         reaction.users.remove(request.user)
+        if not reaction.users.exists():
+            reaction.delete()
         return drf.response.Response(status=status.HTTP_204_NO_CONTENT)