machinectl: don't return 1 in case we couldn't figure out container IP addresses (#5587)

This is in spirit very similar to commit
4b2419165c.

Fixes: #5581

src/machine/machinectl.c

@@ -340,6 +340,7 @@ static int list_machines(int argc, char *argv[], void *userdata) {
                         printf("No machines.\n");
         }
 
+        r = 0;
 out:
         clean_machine_info(machines, n_machines);
         return r;