Fix list comparison for empty list

Fix list comparison for empty list

Replaced boolean expression with 'not fields' in "_filter" method.
Also updated unit test cases which calls "_filter" method with
expected value of fields in restore methods.

The unit test "test_restore_vol_meta" was failing for empty list
because after updating the volume metadata, container was passed
to the "_restore_vol_meta" method where "metadata" is expected.
So fixed this unit test by passing the expected value to the
"_restore_vol_meta" method.

Closes-Bug: 1482619
Change-Id: I95c3a390b0b75c0514a3d63699367081440bf414