Ensure jsToXliff functions generate valid xliff files

[AdriAt360]

Fixes #57

The jsToXliff libs create xliff with wrong format in case of falsy source, e.g. "". In this situation, the generated xliff contains no source tag, which is not valid against the specification,

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• documentation is changed or added -> no change needed for this fix, the doc is accurate

[adrai]

thank you... it's included in v6.2.1

[AdriAt360]

@adrai thanks a lot for your reactivity here 🙏🏽