There are a number of places that improperly handle a set that has
multiple NULL values in it:
 * INSERT on single values
 * UPDATE on single values
 * assert_single

The assert_single case is a long standing bug that can be triggered
without much trouble. The INSERT/UPDATE bugs are new, and stem from
the new ?? on DML, since I think it that was the first thing in the
compiler backend where extra NULLS might be produced for something
with single cardinality.

Fix this by adding null checks at the consumer sites. One of the
consumer sites is set_as_subquery, and I modified update to always use
that.
I also added a helper function to add the null checks, and I'll be
going back and changing lots of other code to use the helper.

We could also flip this around a bit and try saying that we shouldn't
ever produce extra NULLs in a single set, and try to stamp it out on
the producer side, though that seems a little less consistent to me?

Fixes #6438.

this will make it easier to inject the filter on the backend