NULL instead of empty string in response

[Anger79]

I use ClickHouse FDW in a non-standard way, instead of Clickhouse DB I use my own http server that returns TSV formatted data to FDW.
I expect response to contain some empty string fields, but I'm getting NULL instead.

So is there any kind of config option in FDW to/not to treat empty string as NULL or is there any special ouput format for empty strings, not just "" in ClickHouse?