[common] Make sure self.params.get('sleep_interval_subtitles') is int

This can happen if another software is using yt-dlc'API (ie: tubeup).
The stack trace would be:
$ tubeup 'https://youtube.com/watch?v=JyE9OF03cao'
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dlc version 2020.10.25
[debug] Python version 3.7.3 (CPython) - Linux-5.8.0-0.bpo.2-amd64-x86_64-with-debian-10.6
[debug] exe versions: ffmpeg 3.3.9, ffprobe 3.3.9
[debug] Proxy map: {}
There are no annotations to write.
ERROR: '>' not supported between instances of 'NoneType' and 'int'
Traceback (most recent call last):
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/YoutubeDL.py", line 846, in extract_info
    return self.process_ie_result(ie_result, download, extra_info)
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/YoutubeDL.py", line 901, in process_ie_result
    return self.process_video_result(ie_result, download=download)
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/YoutubeDL.py", line 1696, in process_video_result
    self.process_info(new_info)
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/YoutubeDL.py", line 1894, in process_info
    dl(sub_filename, sub_info, subtitle=True)
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/YoutubeDL.py", line 1866, in dl
    return fd.download(name, info, subtitle)
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/downloader/common.py", line 367, in download
    if self.params.get('sleep_interval_subtitles') > 0:
TypeError: '>' not supported between instances of 'NoneType' and 'int'

youtube_dlc/downloader/common.py

@@ -364,8 +364,10 @@ class FileDownloader(object):
                         else '%.2f' % sleep_interval))
                 time.sleep(sleep_interval)
         else:
-            if self.params.get('sleep_interval_subtitles') > 0:
+            sleep_interval_sub = 0
+            if type(self.params.get('sleep_interval_subtitles')) is int:
                 sleep_interval_sub = self.params.get('sleep_interval_subtitles')
+            if sleep_interval_sub > 0:
                 self.to_screen(
                     '[download] Sleeping %s seconds...' % (
                         sleep_interval_sub))