message: fix: $off should ignore undefined handler argument
  fix
diff: @@ -95,6 +95,7 @@ export function eventsMixin (Vue: Class<Component>) {
  vm._events[event] = null
  return vm
  }
  + if (fn) {
  // specific handler
  let cb
  let i = cbs.length
  @@ -105,6 +106,7 @@ export function eventsMixin (Vue: Class<Component>) {
  break
  }
  }
  + }
  return vm
  }
  