Add null check for StringIO in case of line execution timeout.

[huaiyan]

Description

This pull request includes a change to the write method in the log_manager.py file. The change adds a check for None to avoid errors when the line execution timeout is reached and all running nodes are cancelled. This is necessary because for synchronous tools running in a worker thread, they can't be stopped and the context won't change in the worker thread as it's a thread-local variable.

All Promptflow Contribution checklist:

• The pull request does not introduce [breaking changes].
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.
• Create an issue and link to the pull request to get dedicated review from promptflow team. Learn more: suggested workflow.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

SDK CLI Global Config Test Result devs/robbenwang_null_io

2 tests   2 ✅  44s ⏱️
1 suites  0 💤
1 files    0 ❌

Results for commit c81a6c7.

♻️ This comment has been updated with latest results.

[github-actions]

promptflow-tracing unit test result devs/robbenwang_null_io

 4 files   4 suites   22s ⏱️
 4 tests  4 ✅ 0 💤 0 ❌
16 runs  16 ✅ 0 💤 0 ❌

Results for commit c81a6c7.

♻️ This comment has been updated with latest results.

[github-actions]

promptflow-tracing e2e test result devs/robbenwang_null_io

 4 files   4 suites   1m 40s ⏱️
 8 tests  8 ✅ 0 💤 0 ❌
32 runs  32 ✅ 0 💤 0 ❌

Results for commit c81a6c7.

♻️ This comment has been updated with latest results.

[github-actions]

promptflow SDK CLI Azure E2E Test Result devs/robbenwang_null_io

  4 files    4 suites   3m 49s ⏱️
168 tests 149 ✅ 19 💤 0 ❌
672 runs  596 ✅ 76 💤 0 ❌

Results for commit c81a6c7.

♻️ This comment has been updated with latest results.

[github-actions]

Executor Unit Test Result devs/robbenwang_null_io

754 tests   754 ✅  3m 50s ⏱️
  1 suites    0 💤
  1 files      0 ❌

Results for commit c81a6c7.

♻️ This comment has been updated with latest results.

[github-actions]

Executor E2E Test Result devs/robbenwang_null_io

210 tests   208 ✅  6m 32s ⏱️
  1 suites    2 💤
  1 files      0 ❌

Results for commit c81a6c7.

♻️ This comment has been updated with latest results.

[github-actions]

SDK CLI Test Result devs/robbenwang_null_io

   12 files     12 suites   40m 32s ⏱️
  452 tests   434 ✅ 18 💤 0 ❌
1 808 runs  1 736 ✅ 72 💤 0 ❌

Results for commit c81a6c7.

♻️ This comment has been updated with latest results.