Allow 'no result' caching in brightcove_load_video()

If the getVideo() fails in the code below, nothing is cached.

This 'no result' state should be cached (temporarily), to prevent continual API requests for invalid videos.

    $video = NULL;
    brightcove_try(function () use ($client_entity, $video_id, &$video) {
      /** @var \Brightcove\API\CMS $cms */
      list($cms,) = brightcove_create_classes($client_entity);
      $video = $cms->getVideo($video_id);
    });
    if ($video) {
      brightcove_cache_set($cid, $video);
      return $video;
    }