fix: preserve zero values in XML export

[gr8man]

Description
In BaseUtils::getXMLFromResult(), database values of 0 or '0' were incorrectly treated as empty and replaced with empty strings ('') in the generated XML export due to the use of the empty() check.

This PR replaces empty($val) with a strict comparison ($val === null || $val === '') to ensure that zero values are preserved in the exported XML, while still correctly replacing actual null and empty string values with an empty element value.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

Looks good.

Please rebase:

git rebase upstream/develop

Update PHPStan baseline:

composer phpstan:baseline

And later, push changes:

git push --force-with-lease origin fix/xml-export-zero-value