[Bug](exchange) avoid coredump coz nullptr

[BiteTheDDDDt]

What problem does this PR solve?

In some cases (such as when the operator is triggered to cancel before opened), the operator close may encounter a core dump due to a nullptr.

*** Current BE git commitID: 50e6e413f5 ***
*** SIGSEGV unknown detail explain (@0x0) received by PID 3836583 (TID 3840122 OR 0x7f840373a700) from PID 0; stack trace: ***
 0# doris::signal::(anonymous namespace)::FailureSignalHandler(int, siginfo_t*, void*) at /home/zcp/repo_center/doris_enterprise/doris/be/src/common/signal_handler.h:421
 1# os::Linux::chained_handler(int, siginfo_t*, void*) in /data1/doris/java8/jre/lib/amd64/server/libjvm.so
 2# JVM_handle_linux_signal in /data1/doris/java8/jre/lib/amd64/server/libjvm.so
 3# signalHandler(int, siginfo_t*, void*) in /data1/doris/java8/jre/lib/amd64/server/libjvm.so
 4# 0x00007F8FC93CD880 in /lib64/libc.so.6
 5# doris::pipeline::ExchangeSinkLocalState::close(doris::RuntimeState*, doris::Status) at /home/zcp/repo_center/doris_enterprise/doris/be/src/pipeline/exec/exchange_sink_operator.cpp:759
 6# doris::pipeline::DataSinkOperatorXBase::close(doris::RuntimeState*, doris::Status) at /home/zcp/repo_center/doris_enterprise/doris/be/src/pipeline/pipeline_x/operator.h:664
 7# doris::pipeline::PipelineXTask::close(doris::Status, bool) at /home/zcp/repo_center/doris_enterprise/doris/be/src/pipeline/pipeline_x/pipeline_x_task.cpp:462
 8# doris::pipeline::_close_task(doris::pipeline::PipelineTask*, doris::pipeline::PipelineTaskState, doris::Status) at /home/zcp/repo_center/doris_enterprise/doris/be/src/pipeline/task_scheduler.cpp:246
 9# doris::pipeline::TaskScheduler::_do_work(unsigned long) at /home/zcp/repo_center/doris_enterprise/doris/be/src/pipeline/task_scheduler.cpp:288
10# doris::ThreadPool::dispatch_thread() in /data1/doris/be/lib/doris_be
11# doris::Thread::supervise_thread(void*) at /home/zcp/repo_center/doris_enterprise/doris/be/src/util/thread.cpp:499
12# start_thread in /lib64/libpthread.so.0
13# clone in /lib64/libc.so.6

This pull request makes a small improvement to the ExchangeSinkLocalState::close method by adding a null check before updating channel timers, which helps prevent potential crashes or undefined behavior.

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[BiteTheDDDDt]

run buildall

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 100.00% (4/4) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.43% (24048/33668)
Line Coverage	57.86% (250306/432604)
Region Coverage	53.09% (208360/392494)
Branch Coverage	54.75% (89402/163299)

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

The base branch was changed.

[BiteTheDDDDt]

run buildall

[hello-stephen]

BE UT Coverage Report

Increment line coverage 0.00% (0/4) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	39.14% (10400/26568)
Line Coverage	30.13% (86252/286285)
Region Coverage	27.67% (49780/179936)
Branch Coverage	24.23% (24430/100826)