Add test cases for improving test coverage of release notes tools generator

[chandankumar4]

What would you like to be added (User Story)?

Add Unit test case for improving test coverage for release notes tools

Detailed Description

Reference: #10931 (comment)

Anything else you would like to add?

No response

Label(s) to be applied

/area testing
One or more /area label. See https://github.com/kubernetes-sigs/cluster-api/labels?q=area for the list of labels.

[sbueringer]

/triage accepted

[sbueringer]

/help

[k8s-ci-robot]

@sbueringer:
This request has been marked as needing help from a contributor.

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