Implement null / empty string handling for sqllogictest

[alamb]

As explained in https://duckdb.org/dev/sqllogictest/result_verification#null-values-and-empty-strings

Empty lines have special significance for the SQLLogic test runner: they signify an end of the current statement or query. For that reason, empty strings and NULL values have special syntax that must be used in result verification. NULL values should use the string NULL, and empty strings should use the string (empty), e.g.:

query II
SELECT NULL, ''
----
NULL
(empty)

The goal of this ticket is to make the DataFusion sqlogictest framework do the same thing

[alamb]

Currently the sqllogictest framework uses the arrow csv writer

This we would need some way to control how NULLs are printed in the CSV writer in order to change the display of null values to "NULL" -- I will attempt to upstream that.

Rewriting "" to (empty) is possible now