message: Fix querying for null in a nullable indexed string column
diff: @@ -420,6 +420,12 @@ private:
  void IndexArray::index_string_all_ins(StringData value, IntegerColumn& result, ColumnBase* column) const
  {
  + if (value.is_null()) {
  + // we can't use case_map on null strings because it currently returns an
  + // empty string ("") in that case which is different than a null StringData
  + return index_string_all(value, result, column);
  + }
  +
  const util::Optional<std::string> upper_value = case_map(value, true);
  const util::Optional<std::string> lower_value = case_map(value, false);
  SearchList search_list(upper_value, lower_value);
  