fix: skip invalid source_node_id in end stream generator router

[laipz8200]

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• This change requires a documentation update, included: Dify Document
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• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods