fix: allow None values in VariableMessage validation

[krap730]

Summary

Fix workflow-as-tool invocation failing when output variables contain null values.

Problem

When calling a workflow as a tool, if any output variable has a null value, the tool invocation fails with:

1 validation error for VariableMessage
Value error, Only basic types and lists are allowed.

Root Cause

In api/core/tools/entities/tool_entities.py, the VariableMessage.transform_variable_value validator only allows dict | list | str | int | float | bool types, but does not allow None.

When a workflow End node outputs contain null values (e.g., {"result": "data", "optional_field": null}), the WorkflowTool._invoke() method passes these None values to create_variable_message(), which fails the validation.

Solution

Update the validation to allow None values by checking if value is not None before the isinstance check.

Related to #30069

Test Plan

• Create a workflow with multiple output variables where some may be null
• Call the workflow as a tool from another workflow or agent
• Verify that null output values no longer cause validation errors

[gemini-code-assist]

Summary of Changes

Hello @krap730, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves an issue where workflow-as-tool invocations would fail if any output variable contained a null value. The change updates the VariableMessage validation logic to correctly handle None types, ensuring robust and error-free execution of workflows that produce optional or empty outputs.

Highlights

• VariableMessage Validation: The transform_variable_value method in VariableMessage now explicitly allows None values, preventing workflow-as-tool invocations from failing when output variables are null.

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[krap730]

Note: This issue can also be worked around by ensuring that workflow outputs don't contain null values (e.g., by providing default values in the workflow itself).

Whether to merge this fix is up to the maintainers' discretion - the workaround is a valid alternative if you prefer to keep the stricter validation.

[gemini-code-assist]

Code Review

This pull request resolves a validation error that occurred when a workflow used as a tool returned null output variables. The change in api/core/tools/entities/tool_entities.py correctly updates the VariableMessage.transform_variable_value validator to permit None values, which was the root cause of the issue. The implementation is clean, and the accompanying updates to the docstring and error message are appreciated for maintaining code clarity. The fix is solid and I see no issues.