`"media_files": {"enabled": False}` doesn't serialize out

[fenekku]

Placing "media_files": {"enabled": False} in the conftest.py:input_data makes tests/services/test_record_service.py:test_create_draft and test_publish_draft fail because "media_files": {"enabled": False} is not serialized out, only ``"media_files": {}` is.

"media_files": {"enabled": False} is expected to be serialized out like "files": {"enabled": False} is.

This may be a problem to fix in invenio-records-resources actually, but it was experienced here and didn't have time to dig into it.