Only swap in a thumbnail for videos if a thumbnail is successfully returned

Signed-off-by: Andrew Welch <andrew@nystudio107.com>

Author: khalwat

src/Transcoder.php

@@ -156,7 +156,10 @@ function (AssetThumbEvent $event) {
                 /** @var Asset $asset */
                 $asset = $event->asset;
                 if (AssetsHelper::getFileKindByExtension($asset->filename) === Asset::KIND_VIDEO) {
-                    $event->path = Transcoder::$plugin->transcode->handleGetAssetThumbPath($event);
+                    $path = Transcoder::$plugin->transcode->handleGetAssetThumbPath($event);
+                    if (!empty($path)) {
+                        $event->path = $path;
+                    }
                 }
             }
         );