umount: be happy if /proc/swaps doesn't exist

Kernels work without swap just fine.

Fixes: #13993

src/shutdown/umount.c

@@ -184,6 +184,8 @@ int swap_list_get(const char *swaps, MountPoint **head) {
                 return log_oom();
 
         r = mnt_table_parse_swaps(t, swaps);
+        if (r == -ENOENT) /* no /proc/swaps is fine */
+                return 0;
         if (r < 0)
                 return log_error_errno(r, "Failed to parse %s: %m", swaps ?: "/proc/swaps");