[Validator] Minor fix in XmlLoader
bug #10119 [Validator] Minor fix in XmlFileLoader (florianv)

This PR was merged into the 2.3 branch.

Discussion
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[Validator] Minor fix in XmlFileLoader

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

An instance of `\SimpleXMLElement` is passed to the `newConstraint` method instead of a string which can lead to weird type juggling issues.

Commits
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16b726c [Validator] Minor fix in XmlLoader