fix: DynamicCommunitySelection crashes with KeyError when no communities at level 0

[hanhan761]

Fixes #2382

Problem

DynamicCommunitySelection.__init__ at dynamic_community_selection.py:71 accesses self.levels["0"] directly. When the input data contains no communities at level "0" (e.g., all communities start at level "1" or the list is empty), this raises KeyError: '0' and crashes the dynamic community selection flow.

self.levels is built from the communities list — if no community has level="0", the key is never added.

Fix

Replace self.levels["0"] with self.levels.get("0", []) so that starting_communities defaults to an empty list when no root-level communities exist.

Test

Added test_dynamic_community_selection_no_level_zero which creates communities only at levels 1 and 2 (no level 0) and verifies:

• The constructor does not raise KeyError
• selector.starting_communities is an empty list
• "0" is absent from selector.levels