support breaking changes without a longer description

Currently if I try to create a commit with breaking changes but without a longer description I get the following message: "A BREAKING CHANGE commit requires a body. Please enter a longer description of the commit itself:". According to the [examples](https://www.conventionalcommits.org/en/v1.0.0-beta.2/#commit-message-with-description-and-breaking-change-in-body) from the Conventional Commits spec commit messages where the body consists of only a breaking change are valid and should be supported by Commitizen.