DijkstraPathPlanning returns a destination-only path when no path exists #160

Problem

When a destination is unreachable, DijkstraPathPlanning.get_path() returns [to_entity_id] instead of an empty path. Consequently, get_distance() returns 0.0, making an unreachable destination indistinguishable from a valid zero-distance path.

Expected behavior

• An unreachable destination returns [] from get_path().
• Its distance is float("inf").
• A path from an entity to itself still has distance 0.0.