Implement null / empty string handling for sqllogictest #4500
Description
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