[FEATURE]: Allow undefined inside union()

### Describe what you want

It should be possible to do `union(query1, query2, undefined)`.

This would enable conditional logic in the same vein it's possible to use it in filters (e.g. `where(and(eq(id, 1), undefined))`

A side requirement of allowing `undefined` inside `union()` would be to allow passing just 1 argument to `union()`. Right now, it expects 2.