fix: Handle fallback video format on CourseUnitNavigationActivity (#1835)

- Show fallback video in case course doesn't have any video quality available
fixes: LEARNER-9658

Author: omerhabib26

OpenEdXMobile/src/main/java/org/edx/mobile/model/course/VideoBlockModel.java

@@ -77,9 +77,11 @@ public void setData(VideoData data) {
      * @return The size of the video if available, <code>-1</code> otherwise.
      */
     public long getPreferredVideoEncodingSize(VideoQuality preferredVideoQuality) {
-        if (data != null && data.encodedVideos != null
-                && data.encodedVideos.getPreferredVideoInfoForDownloading(preferredVideoQuality) != null) {
-            return data.encodedVideos.getPreferredVideoInfoForDownloading(preferredVideoQuality).fileSize;
+        if (data != null && data.encodedVideos != null) {
+            VideoInfo videoInfo = data.encodedVideos.getPreferredVideoInfoForDownloading(preferredVideoQuality);
+            if (videoInfo != null && videoInfo.fileSize > 0) {
+                return videoInfo.fileSize;
+            }
         }
         return -1;
     }

OpenEdXMobile/src/main/java/org/edx/mobile/module/db/DatabaseModelFactory.java

@@ -103,9 +103,15 @@ public static VideoModel getModel(VideoData vrm, VideoBlockModel block, VideoQua
         e.eid = root.getCourseId();
         e.duration = vrm.duration;
         final VideoInfo preferredVideoInfo = vrm.encodedVideos.getPreferredVideoInfoForDownloading(videoQuality);
-        e.size = preferredVideoInfo.fileSize;
+        final VideoInfo videoInfo = vrm.encodedVideos.getPreferredNativeVideoInfo();
+        if (preferredVideoInfo != null) {
+            e.size = preferredVideoInfo.fileSize;
+            e.url = preferredVideoInfo.url;
+        } else if (videoInfo != null) {
+            e.size = videoInfo.fileSize;
+            e.url = videoInfo.url;
+        }
         e.title = block.getDisplayName();
-        e.url = preferredVideoInfo.url;
         e.url_hls = getVideoNetworkUrlOrNull(vrm.encodedVideos.getHls());
         e.url_high_quality = getVideoNetworkUrlOrNull(vrm.encodedVideos.getMobileHigh());
         e.url_low_quality = getVideoNetworkUrlOrNull(vrm.encodedVideos.getMobileLow());