Don't return early if a changelog entry is present but validate it

Currently the danger-bot will skip the changelog check if the MR does not require a changelog entry per the rules defined here: https://gitlab.com/gitlab-org/security-products/danger-bot/-/blob/b32df577b012d49b8d7ba95821c97a02eddc317e/changelog/changelog.rb#L23-25

Though, even if optional, a changelog entry should always be checked if it is provided.