small tweak to bio_feature types file to ensure that at least an empty string is returned so no exception thrown because trying a string function on None
small tweak to bio_feature types file to ensure that at least an empty string is returned so no exception thrown because trying a string function on None

4dn-dcic/fourfront,hms-dbmi/fourfront,hms-dbmi/fourfront,hms-dbmi/fourfront,4dn-dcic/fourfront,4dn-dcic/fourfront,hms-dbmi/fourfront,hms-dbmi/fourfront,4dn-dcic/fourfront