[fix](group commit) fix NPE in group commit select backend

[mymeiyi]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

2024-11-11 12:00:12,200 WARN (mysql-nio-pool-24123|259) execute Exception. stmt[15559118, 7d88d2401de44f4b-8325fbee3de50791]
java.lang.NullPointerException: Cannot invoke "org.apache.doris.system.Backend.isActive()" because "backend" is null
        at org.apache.doris.load.GroupCommitManager.getCachedBackend(GroupCommitManager.java:305) ~[doris-fe.jar:1.2-SNAPSHOT]
        at org.apache.doris.load.GroupCommitManager.selectBackendForCloudGroupCommitInternal(GroupCommitManager.java:251) ~[doris-fe.jar:1.2-SNAPSHOT]
        at org.apache.doris.load.GroupCommitManager.selectBackendForGroupCommitInternal(GroupCommitManager.java:239) ~[doris-fe.jar:1.2-SNAPSHOT]
        at org.apache.doris.load.GroupCommitManager.selectBackendForGroupCommit(GroupCommitManager.java:203) ~[doris-fe.jar:1.2-SNAPSHOT]
        at org.apache.doris.cloud.planner.CloudGroupCommitPlanner.selectBackends(CloudGroupCommitPlanner.java:49) ~[doris-fe.jar:1.2-SNAPSHOT]

Release note

None

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 : drop backend is not supported in regression and it's a simple fix

• 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

[doris-robot]

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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.
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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?

[mymeiyi]

run buildall

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.

[dataroaring]

LGTM