fix: return None for null Eigen matrix pointers

[jad-fahmi]

Description

Fixes #5635.

The dense Eigen caster passed null pointers to capsule creation or dereferenced them, depending on the return policy. Return None before applying the policy, matching other pointer casters. The regression covers mutable and const matrix pointers with all seven return policies.

Validation with Eigen 3.4.0:

• On bf21a6df with the regression tests added, python3 -m pytest /baseline/tests/test_eigen_matrix.py -k 'null_matrix and automatic and not reference' -q fails both cases with ValueError: PyCapsule_New called with null pointer.
• Linux, GCC 13.3, C++11, Python 3.12: python3 -m pytest /baseline/tests/test_eigen_matrix.py -q passes all 46 tests. cmake --build /build --target check -j 6 passes: 1,328 Python tests (48 skipped), standalone C++ tests, embedded-interpreter tests, cross-module RTTI tests, and six CMake integration checks.
• Windows, GCC 14.2, Python 3.13: the Eigen matrix suite passes all 46 tests using python -c "import os, pytest; dlls = os.add_dll_directory('C:/msys64/ucrt64/bin'); raise SystemExit(pytest.main(['../../tests/test_eigen_matrix.py', '-q']))" from build/tests.
• prek run --all-files --quiet and git diff --check pass.

Suggested changelog entry:

• Return None when a bound function returns a null pointer to a dense Eigen matrix.