refactor: Handle missing UNRELEASED sections and improve flexibility #289

Job	Run time
cli-integration (macos)	2m 33s
cli-integration (windows)	26s
cli-integration (ubuntu)	13s
updater-create-pr / cancel-previous-run	9s
updater-test-args / cancel-previous-run	5s
updater-test-args / validate-inputs	8s
updater-create-pr / validate-inputs	8s
updater-create-pr / update	28s
updater-test-args / update	11s
updater-test-outputs	4s
	4m 25s

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