Fix lazyproperty re-evaluating getters that return None - #1626

Fixes #1600.

lazyproperty treated a cached None the same as "not computed yet", so a getter that legitimately returns None ran again on every access. That contradicts the documented "evaluated only on first access" behavior.

The lookup now uses key presence in the instance __dict__ instead of comparing the stored value to None.

Testing

• uv run pytest tests/test_shared.py — 23 passed
• New None-returning case fails on unmodified master (call_count 3 vs 1) and passes with this change
• Existing non-None caching still holds
• ruff check / ruff format --check on the changed files
• Full uv run pytest on Windows: 1555 passed, 56 failed. The failures are existing XML snippet / SHA-1 fixtures (tests/oxml/test_table.py, tests/opc/test_phys_pkg.py, header/footer templates) from CRLF checkout; they are outside the files in this PR.