fix: handling case of stix events without timestamp
fix: handling case of stix events without timestamp

SteveClement/MISP,MISP/MISP,devnull-/MISP,TheDr1ver/MISP,FloatingGhost/MISP,FloatingGhost/MISP,FloatingGhost/MISP,FloatingGhost/MISP,devnull-/MISP,SteveClement/MISP,Rafiot/MISP,devnull-/MISP,TheDr1ver/MISP,MISP/MISP,FIRSTdotorg/MISP,FIRSTdotorg/MISP,TheDr1ver/MISP,SteveClement/MISP,FIRSTdotorg/MISP,TheDr1ver/MISP,MISP/MISP,MISP/MISP,FIRSTdotorg/MISP,MISP/MISP,Rafiot/MISP,Rafiot/MISP,devnull-/MISP,SteveClement/MISP,Rafiot/MISP,SteveClement/MISP