fix: Use "undetermined" for missing CC language

When caption languages are not present in the manifest (such as
"CC2=;CC3="), use "und", the language code for "undetermined".

Bug: 187442669
Change-Id: Ia129815efb2ae6b0ac6b655ac02398a7b1f70e68

lib/dash/dash_parser.js

@@ -863,7 +863,9 @@ shaka.dash.DashParser = class {
               channel = channelAndLanguage[0].startsWith('CC') ?
                   channelAndLanguage[0] : `CC${channelAndLanguage[0]}`;
 
-              language = channelAndLanguage[1]; // 3 letters (ISO 639-2)
+              // 3 letters (ISO 639-2).  In b/187442669, we saw a blank string
+              // (CC2=;CC3=), so default to "und" (the code for "undetermined").
+              language = channelAndLanguage[1] || 'und';
             }
             closedCaptions.set(channel, LanguageUtils.normalize(language));
           }