add nil check in json_unquote function as LongText.Convert() can retu…

[iluminae]

…rn nil,nil

This fixes an issue that can trigger a panic in master with:

SELECT json_unquote(json_extract('{"hi":"there"}', '$.nope'))

When json_extract returns with a path that is not resolved in the result, json_unquote will blow up while producing an error.

[zachmu]

Thanks for the contribution!

Can you add an engine test that fails before your fix and passes after? Probably should go in queries.go, in the enginetest package.

[zachmu]

hi @iluminae,

I'm going to close this out due to inactivity. Please feel free to resubmit if you can add the engine test as asked, we would be happy to get the contribution.