Union columns can never be `NULL` (I think?)

[samuelcolvin]

Describe the bug

Maybe I'm doing something wrong with my union in datafusion-functions-json, but is null expressions never evaluate to true with the JsonUnion column.

They behave as expected when the function in question returns a scalar JsonUnion.

To Reproduce

See https://github.com/datafusion-contrib/datafusion-functions-json/pull/24

Expected behavior

When the value in every column of the union is null:

• <union column> is null should evaluate to null
• I guess it would also be nice if the repr of null values in a union was an empty string same as everything else, not {<first column_name>=} (which I what I think it is now)

Additional context

No response