Fix/index error when unhighlighting

[magdaaniol]

Description

The logic in unhighlight_ents_in_doc does not account for the fact that there might exist unhighlighted entities in doc, which will lead to IndexError if that's the case.

Corresponding documentation PR

Types of change

Bug fix

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran all tests in tests and usage_examples/tests, and all new and existing tests passed. This includes
  • all external tests (i. e. pytest ran with --external)
  • all tests requiring a GPU (i. e. pytest ran with --gpu)
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[rmitsch]

Thanks for the fix! 🎉