[outtmpl] Handle `-o ""` better

Since the specific type of file is not downloaded when giving `-o "<type>:"`,
now `-o ""` acts as an alias to `--skip-download`

README.md

@@ -1137,7 +1137,7 @@ To summarize, the general syntax for a field is:
 %(name[.keys][addition][>strf][,alternate][&replacement][|default])[flags][width][.precision][length]type
 ```
 
-Additionally, you can set different output templates for the various metadata files separately from the general output template by specifying the type of file followed by the template separated by a colon `:`. The different file types supported are `subtitle`, `thumbnail`, `description`, `annotation` (deprecated), `infojson`, `link`, `pl_thumbnail`, `pl_description`, `pl_infojson`, `chapter`, `pl_video`. For example, `-o "%(title)s.%(ext)s" -o "thumbnail:%(title)s\%(title)s.%(ext)s"`  will put the thumbnails in a folder with the same name as the video. If any of the templates (except default) is empty, that type of file will not be written. Eg: `--write-thumbnail -o "thumbnail:"` will write thumbnails only for playlists and not for video.
+Additionally, you can set different output templates for the various metadata files separately from the general output template by specifying the type of file followed by the template separated by a colon `:`. The different file types supported are `subtitle`, `thumbnail`, `description`, `annotation` (deprecated), `infojson`, `link`, `pl_thumbnail`, `pl_description`, `pl_infojson`, `chapter`, `pl_video`. For example, `-o "%(title)s.%(ext)s" -o "thumbnail:%(title)s\%(title)s.%(ext)s"`  will put the thumbnails in a folder with the same name as the video. If any of the templates is empty, that type of file will not be written. Eg: `--write-thumbnail -o "thumbnail:"` will write thumbnails only for playlists and not for video.
 
 The available fields are:
 

yt_dlp/__init__.py

@@ -335,6 +335,9 @@ def _real_main(argv=None):
     if _video_multistreams_set is False and _audio_multistreams_set is False:
         _unused_compat_opt('multistreams')
     outtmpl_default = opts.outtmpl.get('default')
+    if outtmpl_default == '':
+        outtmpl_default, opts.skip_download = None, True
+        del opts.outtmpl['default']
     if opts.useid:
         if outtmpl_default is None:
             outtmpl_default = opts.outtmpl['default'] = '%(id)s.%(ext)s'