[common] Fix FourCC fallback when parsing ISM (#372)

In some DASH manifests, the FourCC attribute is actually present,
but empty.  We thus apply the same fallback to 'AACL' that we do
when the attribute is entirely absent.

Authored by: fstirlitz

yt_dlp/extractor/common.py

@@ -2876,7 +2876,7 @@ class InfoExtractor(object):
             stream_name = stream.get('Name')
             stream_language = stream.get('Language', 'und')
             for track in stream.findall('QualityLevel'):
-                fourcc = track.get('FourCC', 'AACL' if track.get('AudioTag') == '255' else None)
+                fourcc = track.get('FourCC') or ('AACL' if track.get('AudioTag') == '255' else None)
                 # TODO: add support for WVC1 and WMAP
                 if fourcc not in ('H264', 'AVC1', 'AACL', 'TTML'):
                     self.report_warning('%s is not a supported codec' % fourcc)