fix #4466: show snapshots in media content again

while refactoring the the empty media set checks, we accidentally
reversed one check from !is_empty to unassigned, which now never
included the right media sets.

reversing the condition fixes that.

fixes
52517f7b: ("tape: hide internal use of all zero uuid for unassigned tapes")

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/tape/inventory.rs

@@ -294,7 +294,7 @@ impl Inventory {
             match entry.id.media_set_label {
                 None => continue, // not assigned to any pool
                 Some(ref set) => {
-                    if set.unassigned() {
+                    if !set.unassigned() {
                         list.push(entry.id.clone());
                     }
                 }