remove warnings when importing event attributes without distribution set
remove warnings when importing event attributes without distribution set

Warning (2): Illegal string offset 'distribution' [APP/Model/Event.php, line 1810]
Notice (8): Uninitialized string offset: 0 [APP/Model/Event.php, line 1810]
Warning (2): Illegal string offset 'distribution' [APP/Model/Event.php, line 1821]
Notice (8): Uninitialized string offset: 0 [APP/Model/Event.php, line 1821]
FloatingGhost/MISP,Rafiot/MISP,MISP/MISP,SteveClement/MISP,crruthe/MISP,cristianbell/MISP,cristianbell/MISP,TheDr1ver/MISP,FIRSTdotorg/MISP,cristianbell/MISP,cristianbell/MISP,TheDr1ver/MISP,RichieB2B/MISP,FIRSTdotorg/MISP,devnull-/MISP,devnull-/MISP,FIRSTdotorg/MISP,TheDr1ver/MISP,RichieB2B/MISP,TheDr1ver/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP,RichieB2B/MISP,FloatingGhost/MISP,Rafiot/MISP,FloatingGhost/MISP,devnull-/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP,MISP/MISP,FIRSTdotorg/MISP,Rafiot/MISP,MISP/MISP,crruthe/MISP,Rafiot/MISP,RichieB2B/MISP,devnull-/MISP,crruthe/MISP,crruthe/MISP,FloatingGhost/MISP