fix: preserve NaN-level entities in _filter_under_community_level (fixes #2348)

[hanhan761]

Summary

_filter_under_community_level() filters rows with df[df.level <= community_level]. In pandas/NumPy, np.nan <= X evaluates to False, so entities without community assignments (where level is NaN after a left join) are silently dropped from query results.

This fix adds | df.level.isna() to the filter condition, preserving entities that have no community assignment. "Not assigned to any community" is semantically different from "assigned to a community above the requested level."

Issue

Fixes #2348

Verification

• Verified with isolated test: NaN entities now preserved, normal filtering unchanged
• 4 test scenarios pass: NaN preservation, normal filtering, community_level=0 edge case