Add a no-op validator for use in tests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.04%. Comparing base (8ea34d2) to head (1c20c33).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #35 +/- ## ========================================== + Coverage 81.88% 82.04% +0.16% ========================================== Files 16 17 +1 Lines 436 440 +4 ========================================== + Hits 357 361 +4 Misses 55 55 Partials 24 24 ```

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[djjuhasz]

This isn't necessary, I can just call xmlvalidate.New(nil) with a nil argument.