btrfs/179: fix test failure when there are no snapshots to delete

btrfs/179: fix test failure when there are no snapshots to delete

It's very rare, but we can end up in a situation where there are no
snapshots to delete, in which case the $victim variable of the function
delete_workload() ends up being assigned with an empty string. When
that happens we end up running the command:

   btrfs subvolume delete "$SCRATCH_MNT/snapshots/"

Which fails since the argument is not a subvolume or a snapshot.
This causes the test to fail due to an unexpected error message from
the subvolume delete command:

  btrfs/179 129s ... - output mismatch (see /home/fdmanana/git/hub/xfstests/results//btrfs/179.out.bad)
      --- tests/btrfs/179.out	2020-10-16 23:13:46.546152332 +0100
      +++ /home/fdmanana/git/hub/xfstests/results//btrfs/179.out.bad	2021-09-24 11:15:01.404863801 +0100
      @@ -1,2 +1,3 @@
       QA output created by 179
      +ERROR: Not a Btrfs subvolume: Invalid argument
       Silence is golden
      ...

Fix that by making the delete_workload() loop skip the deletion attempt
when there are no snapshots.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>