generate empty file when pass option -w

[bigwhite]

when the source file passed to gen command(with option -w) is like below:

// github.com/panjf2000/internal/netpoll/queue/queue.go
package queue

// Task is a asynchronous function.
type Task func() error

// AsyncTaskQueue is a queue storing asynchronous tasks.
type AsyncTaskQueue interface {
    Enqueue(Task)
    Dequeue() Task
    Empty() bool
}

then，gen command generate an empty queue.go with success hint.

[bigwhite]

the file to add trace has no function or method，so the rewrite func return "nil, nil". we should handle the first nil gracefully.