babel: do not merge NullTranslations

Our homegrown Babel function `get_translation` throws an `AttributeError` when the code tries to `merge` a `NullTranslations` object.

I was seeing this when missing the compiled translations (*.mo) after installing Invenio via overlay. The practical solution was to just compile the translations with `pybabel compile` on the site-packages folder of Invenio.

Dunno if this is the right approach though.. @jirikuncar