pbs: free image: explicitly return undef

Storage.pm's vdisk_free interprets truthy return values as worker subs, so be
explicit about returning undef here. Not an issue at the moment, because
run_client_command already returns undef, but better be safe than sorry.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>

PVE/Storage/PBSPlugin.pm

@@ -595,6 +595,8 @@ sub free_image {
     my ($vtype, $name, $vmid) = $class->parse_volname($volname);
 
     run_client_cmd($scfg, $storeid, "forget", [ $name ], 1);
+
+    return;
 }