Prevent STIX export crash
Prevent STIX export crash

attribute can be None which causes the STIX conversion to crash
MISP/MISP,SteveClement/MISP,MISP/MISP,Rafiot/MISP,MISP/MISP,MISP/MISP,FIRSTdotorg/MISP,FloatingGhost/MISP,devnull-/MISP,devnull-/MISP,SteveClement/MISP,FIRSTdotorg/MISP,MISP/MISP,Rafiot/MISP,FloatingGhost/MISP,SteveClement/MISP,devnull-/MISP,devnull-/MISP,FIRSTdotorg/MISP,FloatingGhost/MISP,FloatingGhost/MISP,Rafiot/MISP,Rafiot/MISP,SteveClement/MISP,SteveClement/MISP,FIRSTdotorg/MISP