function handle$in(val, context) {
  const _this = this;
  if (!Array.isArray(val)) {
    return [this.castForQuery(null, val, context)];
  }
  return val.map(function(m) {
    if (Array.isArray(m) && m.length === 0) {
      return m;
    }
    return _this.castForQuery(null, m, context);
  });
}
Just like handleArray, except also allows `[]` because surprisingly
`$in: [1, []]` works fine
@api private