Return json null for nullable values in http api response data

[everpcpc]

cc @sundy-li

It seems https://github.com/datafuselabs/databend/issues/15960 should be reverted? cc @youngsofun

Server

• [x] https://github.com/datafuselabs/databend/pull/16120

Clients

• [x] bendsql: https://github.com/datafuselabs/bendsql/pull/465
• [x] databend-go: https://github.com/datafuselabs/databend-go/pull/133

[youngsofun]

@everpcpc no need to revert，the old JsonBlock only use json::Value::String, null is output as "NULL" like output format, change it to StringBlock is more clear.

but your PRs are good. the problem is https://github.com/datafuselabs/databend/issues/16076.

[hantmac]

databend-jdbc no need to modify because the data context defined as an object. https://github.com/datafuselabs/databend-jdbc/blob/main/databend-client/src/main/java/com/databend/client/QueryResults.java#L48