nixos/borgbackup: allow paths to be empty or relative (#51275)

This former necessary in order to exclusively use `--pattern` or `--patterns-from`.
Fixes #51267.

nixos/modules/services/backup/borgbackup.nix

@@ -191,7 +191,7 @@ in {
         options = {
 
           paths = mkOption {
-            type = with types; either path (nonEmptyListOf path);
+            type = with types; either path (listOf str);
             description = "Path(s) to back up.";
             example = "/home/user";
             apply = x: if isList x then x else [ x ];