Improper Sankey rendering for certain minimal topologies with loops

Sankey engine fails to correctly draw certain graphs containing loops. Attached is a minimal example containing 4 nodes and 5 traces. If any trace value is set to 0, the graph renders correctly.

Tested in python on plotly 5.5.0

import plotly.graph_objects as go
nodes = {'label': ['node 0', 'node 1', 'node 2', 'node 3']}
links = {'source': [0, 1, 2, 2, 3], 'target': [3, 0, 0, 3, 2], 'value': [1, 1, 1, 1, 1]}
go.Figure(go.Sankey(node=nodes, link=links)).show()

In the correct rendering, the trace connecting node 2 to node 0 should be straight across.

This may be related to https://github.com/plotly/plotly.py/issues/3568

从 issue 中的最小 Python Sankey 示例开始，比较所有链接值均为 1 时与其中一个值为 0 时的渲染结果。然后查看相关 issue #3568 以了解背景，并跟踪 Sankey 的渲染路径，确定为什么从节点 2 到节点 0 的链接不是直线；当所提供的拓扑能够正确渲染时，即视为完成。