fix: 'validation error for ToolInvokeMessage' when blob_message meta is None

[catusax]

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this change fix a issue when custom tool returns a blob message without metadata, a

validation error for ToolInvokeMessage\nmeta\n Input should be a valid dictionary

will be throwed, because meta definition is meta: dict[str, Any] = Field(default_factory=dict) which can not be set to None.

Screenshots

Before:	After:
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Checklist

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• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• 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

[catusax]

u r right! @laipz8200 , i have recovered the function signature

[laipz8200]

Thanks