[fix] Fix ReActAgent failure when output schema is null.

[wenjin272]

Purpose of change

Fix ReActAgent failure when output schema is null

Tests

it

API

no

Documentation

• doc-needed
• doc-not-needed
• doc-included

[weiqingy]

Thanks for tracking this down — omitting the output_schema key when it's null is the right fix (the old null value was what ActionJsonSerializer choked on, and an absent key reads back as null/None just the same), and nice that the Java and Python sides stay symmetric. One question on the new Java test inline.

[weiqingy]

This test passes as long as env.execute() doesn't throw, so it covers "the plan serializes and the job runs" — which is the crash this PR fixes — but it doesn't pin the contract the no-schema path establishes: that the output comes back as a String. The Python counterpart goes a step further with assert len(output_list) == 1 and assert isinstance(output_list[0]["0001"], str), and the schema-case Java test routes through checkResult(...) for an exact value check. Would it be worth collecting the output here too — collectAsync() like the schema test, then asserting hasNext() and that the value is a String — so a regression that emitted nothing or wrongly took the STRUCTURED_OUTPUT branch in stopAction would fail on the Java side as well? Or is the intent to keep the Java e2e deliberately lighter than the Python one?

[wenjin272]

Good catch. I agree that the Java test also needs to validate the results, and I will update this PR accordingly.

Additionally, since this fix has already been merged into the main branch, I will open a separate PR later to update the Java tests on the main branch as well.