Funders ROR v2 failing for label names with lang None

[ptamarit]

Related to #316

How to reproduce the problem:

1. Download the latest ROR Data Dump (version v1.47 at the time of writing).
2. Run the following command:

   $ invenio vocabularies import -v funders -o v1.47-2024-05-30-ror-data.zip | grep ValidationError
   FundersServiceWriter: [{'ValidationError': {'title': defaultdict(<class 'dict'>, {None: {'key': ['Field may not be null.']}})}}]
   FundersServiceWriter: [{'ValidationError': {'title': defaultdict(<class 'dict'>, {None: {'key': ['Field may not be null.']}})}}]
   FundersServiceWriter: [{'ValidationError': {'title': defaultdict(<class 'dict'>, {None: {'key': ['Field may not be null.']}})}}]
   ...