fix: $off should ignore undefined handler argument
fix: $off should ignore undefined handler argument

fix #6591

lizhiyao/vue,lizhiyao/vue,vuejs/vue,Jinjiang/vue,Hanks10100/vue,cnchanning/vue,Kingwl/vue,Ddder-FE/vue,KingMario/vue,cnchanning/vue,KingMario/vue,KingMario/vue,Hanks10100/vue,Kingwl/vue,vuejs/vue,Ddder-FE/vue,optimizely/vue,Jinjiang/vue,lizhiyao/vue,vuejs/vue,Kingwl/vue,Jinjiang/vue,Ddder-FE/vue,Ddder-FE/vue,lizhiyao/vue,KingMario/vue,Hanks10100/vue,cnchanning/vue,vuejs/vue,Hanks10100/vue,Kingwl/vue,Jinjiang/vue