fix(agent): avoid literal null chat output

[zyssyz123]

Summary

• Avoid exposing literal null when Agent backend terminal output is null.
• Preserve already streamed assistant text as the final message content when terminal output is empty/null.
• Add Agent App runner regression tests for both null-output paths.

Testing

• PYTHONPATH=/tmp/dify-no-readline uv run pytest tests/unit_tests/core/app/apps/agent_app -q (81 passed)
• Deployed to deploy/agent (f83cce3846) and verified on https://agent.dify.dev:
  • Created Agent Roster app through /console/api/agent
  • Checked out and saved build draft with model/prompt
  • Ran Build/Preview chat with draft_type=debug_build
  • Applied build draft and published
  • Ran published Agent chat
  • Confirmed streaming responses and /chat-messages history do not contain literal null

[github-actions]

Pyrefly Diff

base → PR

--- /tmp/pyrefly_base.txt	2026-07-02 03:05:48.076983650 +0000
+++ /tmp/pyrefly_pr.txt	2026-07-02 03:05:38.481948688 +0000
@@ -3230,19 +3230,23 @@
 ERROR Argument `DummyAccount` is not assignable to parameter `user` with type `Account | EndUser` in function `core.app.apps.agent_app.app_generator.AgentAppGenerator._run_stateless` [bad-argument-type]
    --> tests/unit_tests/core/app/apps/agent_app/test_app_generator.py:258:18
 ERROR Object of class `NoneType` has no attribute `message` [missing-attribute]
-   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:362:12
+   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:402:12
 ERROR Object of class `NoneType` has no attribute `model` [missing-attribute]
-   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:363:12
+   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:403:12
 ERROR Object of class `NoneType` has no attribute `message` [missing-attribute]
-   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:396:12
+   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:436:12
 ERROR Object of class `NoneType` has no attribute `usage` [missing-attribute]
-   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:397:12
+   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:437:12
 ERROR Object of class `NoneType` has no attribute `usage` [missing-attribute]
-   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:398:12
+   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:438:12
 ERROR Object of class `NoneType` has no attribute `usage` [missing-attribute]
-   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:399:12
+   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:439:12
 ERROR Object of class `NoneType` has no attribute `message` [missing-attribute]
-   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:414:12
+   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:454:12
+ERROR Object of class `NoneType` has no attribute `message` [missing-attribute]
+   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:468:12
+ERROR Object of class `NoneType` has no attribute `message` [missing-attribute]
+   --> tests/unit_tests/core/app/apps/agent_app/test_app_runner.py:482:12
 ERROR Object of class `NoneType` has no attribute `message` [missing-attribute]
   --> tests/unit_tests/core/app/apps/agent_app/test_input_guards.py:67:12
 ERROR Object of class `NoneType` has no attribute `prompt_messages` [missing-attribute]

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	51.86%	51.86%	+0.00%
Strict coverage	51.38%	51.38%	+0.00%
Typed symbols	31,522	31,528	+6
Untyped symbols	29,533	29,535	+2
Modules	2952	2952	0