fix: to_dict() returns None for non-expanded nodes

[gaoflow]

Summary

Tree.to_dict() returns None when called on a node with expanded=False, instead of the expected string (tag) or dict (with children). This also breaks to_json(), which returns "null" for collapsed trees.

The root cause: when self[nid].expanded is False, the method skips the if block entirely and falls through without hitting a return statement, yielding None by default.

Reproduction:

from treelib import Tree
t = Tree()
t.create_node("Root", "root")
t.create_node("A", "a", parent="root")
t["root"].expanded = False

print(t.to_dict())    # None (expected "Root")
print(t.to_json())    # "null" (expected '"Root"')

Fix: Add an else clause to return the tag/dict-with-data when the node is not expanded, consistent with how leaf nodes (expanded=True, no children) are already handled.

All 207 existing tests pass. AI-assisted.

[liamlundy]

@gaoflow can you add a unit test for this case and make sure that all the tests pass?

[gaoflow]

This is already fixed on master — 700d9d7 rewrote to_dict, and the collapsed case now falls through to return ntag instead of dropping off the end.

On current master the repro gives to_dict() == "Root" and to_json() == '"Root"'; on 700d9d7~1 they were None and "null". So there is nothing left for this PR to do and it can be closed.