fix: [stix upload] Removed 'isset' already tested with 'empty' at the same place
fix: [stix upload] Removed 'isset' already tested with 'empty' at the same place

SteveClement/MISP,SteveClement/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP,MISP/MISP,MISP/MISP,MISP/MISP