…llar data

When merging pillar data, if one pillar file is empty (e.g., due to
conditional rendering that produces no output), None values were
overwriting existing non-empty dictionaries.

This fix adds a _filter_none_overwrites function that recursively
removes None values that would overwrite existing non-empty dicts
during the pillar merge process.

- Added _filter_none_overwrites function to salt/pillar/__init__.py
- Applied filter before merging pillar state
- Added test case to test_pillar.py to verify the fix