Implicit tilesets with multiple contents may not be supported

[javagl]

The forum thread at https://community.cesium.com/t/test-gltf-examples-for-3d-tiles-1-1/40264/6 brought up that there might be an issue with implicit tilesets that use multiple contents, suggesting that trying to upgrade https://github.com/CesiumGS/cesium/tree/4dfda79c33b7f743f0661109ad86decf7a1edd0b/Specs/Data/Cesium3DTiles/Implicit/ImplicitMultipleContents causes an error message

ERROR (CLI): The root of the implicit tileset did not define a template URI
    err: {
      "type": "ImplicitTilingError",
      "message": "The root of the implicit tileset did not define a template URI",
      "stack":
          Error: The root of the implicit tileset did not define a template URI
              at ImplicitTraversedTile.getRawContents (/workspaces/3d-tiles-tools/src/tilesets/traversal/ImplicitTraversedTile.ts:362:17)

which should not happen. (The fact that the tileset already is 1.1, and should not need an upgrade does not matter here - it should not cause this error in any case)