fix: [stix import] Additional test if leveraged ttps are not None to avoid issues
fix: [stix import] Additional test if leveraged ttps are not None to avoid issues

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