Fixes: #20098 - Handle empty object_types field in Tag bulk import

[bctiemann]

Fixes: #20098

Handles the case in CSVMultipleContentTypeField.prepare_value where value is null (which only occurs in Tag bulk import, where the object_types field is optional).

[arthanson]

I think it would be cleaner/clearer right at the top of the function to have:

if not value:
    return None

as none of the other processing needs to be done.