[dagster-postgres] [easy] Quick fix in listen/notify logic

Summary:
There was an unclear isinstance call on the notifications yielded from await_pg_notifications as a NoneType check - a better pattern is the is not None check.

I think the reason this was there in the first place is that older versions of the await_pg_notifications code would occasionally yield a integer signal.

Test Plan: bk

Reviewers: rexledesma

Reviewed By: rexledesma

Differential Revision: https://dagster.phacility.com/D7602