dan initially had it so that if `case_dict[date_modified_key] == ""` to just change the value to None. 

since i figured it's useless to have null values in the index, I initially had it so that if the value wasn't a valid date, to just remove it from the dict, but that's what's been resulting in errors. Because some tests rely on the key to still be there when the value is "" i just put that part of the logic back. It was more hassle fixing Dan's test in a sensible way that wouldn't obscure future errors, then to just mirror his old transform logic.