fix: msi auth issue when secret is not provided

[andyzhangx]

What type of PR is this?
/kind bug

What this PR does / why we need it:
There are 4 auth types: Key, SAS, MSI, SPN, and secret is only optional for MSI auth, this PR fixes the secret not found issue when secret is not provided for MSI auth, this bug was introduced by #867

Which issue(s) this PR fixes:

Fixes #926

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Special notes for your reviewer:

Release note:

none

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andyzhangx

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[andyzhangx]

/cherry-pick release-1.21

[k8s-infra-cherrypick-robot]

@andyzhangx: new pull request created: #928

In response to this:

/cherry-pick release-1.21

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[andyzhangx]

/cherry-pick release-1.20

[k8s-infra-cherrypick-robot]

@andyzhangx: new pull request created: #929

In response to this:

/cherry-pick release-1.20

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.