fix handling of nullable XML attributes

As @Tobion pointed out in #11394, true and 1 are valid values in
boolean XML attributes. The XmlFileLoader didn't handle 1 values
properly.
bug #11672 [Routing] fix handling of nullable XML attributes (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Routing] fix handling of nullable XML attributes

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

As @Tobion pointed out in #11394, ``true`` and ``1`` are valid values in boolean XML attributes. The XmlFileLoader didn't handle ``1`` values properly.

Commits
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7b4d4b6 fix handling of nullable XML attributes