Added edge-case test coverage for admin changelist search and ordering

[San the cyberpunk]

This contribution adds several edge-case tests to the admin_changelist suite to ensure robustness in search, ordering, and filtering.

The new tests in tests/admin_changelist/tests.py cover:

1. Search validation with extremely long strings (10,000 characters).
2. Graceful handling of whitespace-only search queries.
3. Correct ordering behavior when integer fields (like 'age') contain NULL values.
4. Proper filtering for Many-to-Many relationships when no associations exist (empty/invalid ID).

All tests will pass against the main branch.

[Simon Charette]

Unless there's a specific problem at play here I don't think this is necessary; we don't add tests for cases that not proven to be problematic.