Fix JSDoc/TS of writeTextToCanvas

[zoran995]

writeTextToCanvas can also return undefined

[cesium-concierge]

Thanks for the pull request @zoran995!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ 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.
• Works (or fails gracefully) in IE11.

[OmarShehata]

Thanks @zoran995! I don't seem to have push access to your fork/branch, I opened #9219 to update CHANGES.md and add a test.