Add <format_tsv_null_representation> or a <format_settings> section for UDFs (of the .XML sort)

I would think since the XML for a UDF can accept TabSeparated that it should also be possible to specify the setting nil too.
In my actual example, I’d like the empty string. If I use settings format_tsv_null_representation='' at the query level, things work.

```

executable
myFunction

String


String

Nullable(Decimal64(9))
TabSeparated

my_script.sh
0
1
throw

```

See discussion here: https://clickhousedb.slack.com/archives/CU478UEQZ/p1701454353401719?thread_ts=1701275038.258609&cid=CU478UEQZ