allow union input to json_get

[samuelcolvin]

This allows queries of the form

select json_get(json_get(json_data, 'foo'), 0) from test

These cases could be simplified, but it'll be much harder/impossible to simplify use of json unions from CTEs or other parts of the query.

Note

The above query would be much more efficient if written:

select json_get(json_data, 'foo', 0) from test

cc @dmontagu

[davidhewitt]

LGTM 👍