Handle cases where there are no btrfs volumes.

roles/debian_server/files/btrfs-scrub

@@ -5,6 +5,9 @@ for mp in $(lsblk --list --noheadings --output fstype,mountpoint | \
             awk '$1 == "btrfs" {print $2}' | \
             sort -u)
 do
+    # In case there are no btrfs volumes, the for loop will execute once with an
+    # empty string for mp. Validate that indeed we're passed a mountpoint.
+    [ -d "$mp" ] || continue
     status="$(btrfs scrub status "$mp" | awk '$1 == "Status:" {print $2}')"
     if [ "$status" = 'running' ]
     then