treat volume and issue values "0" as empty string, see ubtue/DatenProbleme#95 and ubtue/DatenProbleme#107
treat volume and issue values "0" as empty string, see ubtue/DatenProbleme#95 and ubtue/DatenProbleme#107

ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools,ubtue/ub_tools