fix(UI): Fix exception while casting to mismatched player version

If the remote player does not have getChapters(), the proxied call returns undefined.  This led to exceptions when .length was accessed.

Reported as part of issue shaka-project#7546

ui/chapter_selection.js

@@ -119,14 +119,18 @@ shaka.ui.ChapterSelection = class extends shaka.ui.SettingsMenu {
     const currentLocales = this.localization.getCurrentLocales();
     for (const locale of Array.from(currentLocales)) {
       nextLanguage = locale;
-      nextChapters = this.player.getChapters(nextLanguage);
+      // If player is a proxy, and the cast receiver doesn't support this
+      // method, you get back undefined.
+      nextChapters = this.player.getChapters(nextLanguage) || [];
       if (nextChapters.length) {
         break;
       }
     }
     if (!nextChapters.length) {
       nextLanguage = 'und';
-      nextChapters = this.player.getChapters(nextLanguage);
+      // If player is a proxy, and the cast receiver doesn't support this
+      // method, you get back undefined.
+      nextChapters = this.player.getChapters(nextLanguage) || [];
     }
 
     const languageChanged = nextLanguage !== this.chaptersLanguage_;