Fixes #4098.

codehilite passes unknown options through to Pygments' HtmlFormatter. With Pygments 2.20, ilename: null crashes when the formatter escapes the value. MkDocs now normalizes only the explicit codehilite.filename: null case to an empty string, preserving other None values such as linenums: null.

Tests run:

• python -m unittest mkdocs.tests.config.config_options_tests.MarkdownExtensionsTest mkdocs.tests.structure.page_tests.PageTests
• python -m unittest mkdocs.tests.config.config_options_tests mkdocs.tests.structure.page_tests
• python -m ruff check --ignore PLC0415 mkdocs\config\config_options.py mkdocs\tests\config\config_options_tests.py mkdocs\tests\structure\page_tests.py
• git diff --check
• python -m mkdocs build -f --strict with codehilite.filename: and Pygments 2.20.0