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.