Wrap nullable hoisted filter in IsTrue

[angelamayxie]

fixes dolthub/dolt#10070

Filters that evaluate to null in an EXISTS or IN subquery context should be treated the same as false. When hoisted out of the subquery, nullable filters need to be wrapped in an IsTrue expression to ensure that they are still treated the same as false when they evaluate to null.

Not doing so was causing a bug where rows were being dropped from NOT EXISTS if the filter evaluated to null. Since the filter expression was evaluated to null, the NOT expression wrapping it would evaluate to null as well. This caused the filter iterator to reject the row because it did not evaluate to true. Wrapping the filter expression with IsTrue ensures that nulls are then evaluated to false, which will then evaluate to true with NOT.

[jycor]

LGTM