Shutdown the executor service in KubernetesApplicationOperation and prevent NPE

[turboFei]

:mag: Description

Issue References 🔗

As title.

Fix NPE, because the cleanupTerminatedAppInfoTrigger will be set to null. https://github.com/apache/kyuubi/blob/d3520ddbcea96ec55c525600126047c44c7adb35/kyuubi-server/src/main/scala/org/apache/kyuubi/engine/KubernetesApplicationOperation.scala#L269

Also shutdown the ExecutorService when KubernetesApplicationOperation stoped.

Describe Your Solution 🔧

Shutdown the thread executor service and check the null.

Types of changes :bookmark:

• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Test Plan 🧪

Behavior Without This Pull Request :coffin:

Behavior With This Pull Request :tada:

Related Unit Tests

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Checklist 📝

• [x] This patch was not authored or co-authored using Generative Tooling

Be nice. Be informative.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (d3520dd) to head (78c2a6c).

Files with missing lines	Patch %	Lines
...kyuubi/engine/KubernetesApplicationOperation.scala	0.00%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6785 +/- ## ====================================== Coverage 0.00% 0.00% ====================================== Files 687 687 Lines 42439 42447 +8 Branches 5793 5796 +3 ====================================== - Misses 42439 42447 +8 ```

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