[Config] XmlUtils::convertDomElementToArray does not handle '0'

`XmlUtils::convertDomElementToArray` does not handle `0` as a value of a text node, and interprets it as `null`.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | maybe? (if someone is depending on the previous behaviour.)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT