Demonstrate unions can't be null

[samuelcolvin]

Demonstrates #11162 - union columns (and scalars) never match an is null check.

This applies to both sparse and dense unions.

Similarly union columns alwasy match a is not null check.

This is particularly problematic for datafusion-functions-json, where users will often want to use field->'foo' is not null, see datafusion-contrib/datafusion-functions-json#24.

This is soon going to become problematic for us, hence working on it.

I'll add more details on #11162.

[samuelcolvin]

replaced by #11321.