Switch empty value to empty string rather than None
Switch empty value to empty string rather than None

When None is used this is rendered in the HTML as a string, which then when it is returned to the CKAN server becomes 'None' not None.
An empty string represents no value better.

NaturalHistoryMuseum/ckanext-nhm,NaturalHistoryMuseum/ckanext-nhm,NaturalHistoryMuseum/ckanext-nhm