Fix error when no audio information is found.

如果视频里没有音频，会出现异常： ``` Traceback (most recent call last): File "/home/lu4nx/FunClip/funclip/videoclipper.py", line 447, in <module> main() File "/home/lu4nx/FunClip/funclip/videoclipper.py", line 443, in main runner(**kwargs) File "/home/lu4nx/FunClip/funclip/videoclipper.py", line 396, in runner res_text, res_srt, state = audio_clipper.video_recog(file, sd_switch) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/home/lu4nx/FunClip/funclip/videoclipper.py", line 155, in video_recog video.audio.write_audiofile(audio_file) ^^^^^^^^^^^^^^^^^^^^^^^^^^^ AttributeError: 'NoneType' object has no attribute 'write_audiofile' ```