static dummyStreamDB_(type) {
    return {
      id: 0,
      originalId: '',
      groupId: null,
      primary: false,
      type,
      mimeType: '',
      codecs: '',
      language: '',
      originalLanguage: null,
      label: null,
      width: null,
      height: null,
      encrypted: false,
      keyIds: new Set(),
      segments: [],
      variantIds: [],
      roles: [],
      forced: false,
      channelsCount: null,
      audioSamplingRate: null,
      spatialAudio: false,
      closedCaptions: null,
      external: false,
      fastSwitching: false,
      isAudioMuxedInVideo: false,
      baseOriginalId: null,
    };
  }
Create a dummy StreamDB to fill in periods that are missing a certain type,
to avoid failing the general flattening algorithm.  This won't be used for
audio or video, since those are strictly required in all periods if they
exist in any period.

@param {shaka.util.ManifestParserUtils.ContentType} type
@return {shaka.extern.StreamDB}
@private