Treat null value of json the same as other types

[RFSH]

This PR will apply the change described here.

We should stop treating json/jsonb columns differently in read-only modes. So, a null value should be treated like a database null, and therefore, we should honor the show_null annotation for it (instead of forcefully showing the null in a code block).

The other changed mentioned in the comment required more changes, so I decided to not include it in the same PR.