Add handling empty index key that may cause panic issue

[lixiaobing1]

if len(indexKeys) == 0, "return indexKeys[0]" will cause unexpected result.

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This change is

[k8s-bot]

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Can one of the admins verify that this patch is reasonable to test? If so, please reply "ok to test".
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[k8s-bot]

Can one of the admins verify that this patch is reasonable to test? If so, please reply "ok to test".
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[k8s-bot]

Can one of the admins verify that this patch is reasonable to test? If so, please reply "ok to test".
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[caesarxuchao]

ok to test.

[caesarxuchao]

Sorry, I didn't receive any notification regarding the PR. LGTM. Thanks for contributing code!

[k8s-github-robot]

@lixiaobing10051267
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@lixiaobing10051267
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[k8s-bot]

GCE e2e build/test passed for commit a9d6316.

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@lixiaobing10051267
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@k8s-bot test this [submit-queue is verifying that this PR is safe to merge]

[k8s-bot]

GCE e2e build/test passed for commit a9d6316.

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[k8s-github-robot]

Automatic merge from submit-queue