fix(pytest-filler): Remove collected items without parametrized fork

[marioevz]

🗒️ Description

There's some instances where a test function in a module ends up being skipped and the parametrized fork contains a None value which then triggers an exception in function pytest_collection_modifyitems of test_filler.py.

This PR modifies behavior so that the item is removed if it reaches the pytest_collection_modifyitems without a fork value.

Example fill command:

fill -vv --fork=Prague tests/cancun/eip7516_blobgasfee/test_blobgasfee_opcode.py

fails before this PR.

🔗 Related Issues

None

✅ Checklist

• All: Set appropriate labels for the changes.
• All: Considered squashing commits to improve commit history.
• All: Added an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• Tests: All converted JSON/YML tests from ethereum/tests have been added to converted-ethereum-tests.txt.
• Tests: Included the type and version of evm t8n tool used to locally execute test cases: e.g., ref with commit hash or geth 1.13.1-stable-3f40e65.
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.

[danceratopz]

LGTM!