[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`

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'