Fix credit display for hidden imagery layers

[ggetz]

Fixes #11340

To test:

Use this minimal Sandcastle:

const viewer = new Cesium.Viewer("cesiumContainer", {geocoder: false});
viewer.imageryLayers.get(0).show = false;

The Bing maps logo still shows up in Attribution even though it's impossible for it to show up in the scene.

[cesium-concierge]

Thanks for the pull request @ggetz!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[ggetz]

@jjhembd Would you mind taking a look?

[jjhembd]

Thanks @ggetz! The changes make sense, and the credit is fixed on my end.