…che#9183)

When a snapshot is expired as part of a transaction, the manifest list
file(s) should be deleted when the transaction commits. A recent change
(apache#6634) ensured that these files are not deleted when they have also
been committed as part of a transaction, but this breaks the simple
case where no new files are committed. Fix this by not skipping
deletion when the list of committed files is empty.

TESTING:

Extended a unit test to ensure that manifest list files are deleted.
Ran the test without the fix on a branch where apache#6634 was reverted
to show that this is a regression.

…tion when there are no new snapshots (apache#9221)