message: fix: handle null params in migration
diff: @@ -47,7 +47,7 @@ def upgrade():
  session = db.Session(bind=bind)
  for slc in session.query(Slice):
  - params = json.loads(slc.params)
  + params = json.loads(slc.params or "{}")
  params.pop("time_range_endpoints", None)
  slc.params = json.dumps(params)
  