function getHookArgumentsLength(fn) {
  if (isUndef(fn)) {
    return false
  }
  const invokerFns = fn.fns
  if (isDef(invokerFns)) {
    // invoker
    return getHookArgumentsLength(
      Array.isArray(invokerFns) ? invokerFns[0] : invokerFns,
    )
  } else {
    return (fn._length || fn.length) > 1
  }
}
Normalize a transition hook's argument length. The hook may be:
- a merged hook (invoker) with the original in .fns
- a wrapped component method (check ._length)
- a plain function (.length)