fix: return all-NaN pycnophylactic output for empty valid masks

[gaoflow]

Summary

• return the initialized all-NaN surface when no pixels match any pycnophylactic zone
• convert the existing pycnophylactic raises opaque ValueError on all-NaN zones or no matching zone id #3406 strict-xfail regressions into passing tests

Fixes #3406.

Tests

• uv run --with '.[tests]' pytest xrspatial/tests/test_dasymetric.py::TestPycnophylacticEmptyValid -q\n- uv run --with '.[tests]' pytest xrspatial/tests/test_dasymetric.py::TestPycnophylactic xrspatial/tests/test_dasymetric.py::TestPycnophylacticEmptyValid -q

[github-actions]

Hi @gaoflow, thanks for the PR!

Would you mind filing a quick New contributor introduction issue when you get a chance? It helps us point you at issues that fit what you'd like to work on. Most fields are optional.

Reviewing your PR doesn't depend on it, just a friendly nudge.

[brendancol]

hey @gaoflow... thanks for the PR and interest in xarray-spatial. I love to learn more about you, the stack you are using and your background in computers and geography.