The tool does not account for releases with no changes

Currently the tool expects releases to consist only of sections. For instance.

``` markdown
## 0.3.0

**Added**

- Some change

**Changed**

- Another change
```

However, in rare cases we create releases that have no changes. Currently we've implemented it this way:

``` markdown
## 0.3.0

**Changed**

- There are no changes in this release.
```

This is a bit weird, however, and it prevents us from being able to use the `--pr-links` option, as we then have to associate this entry with a PR. It would be great to be able to say:

``` markdown
## 0.3.0

There are no changes in this release.
```

To address this, we could either:

a) allow the body of a release to be either a set of paragraphs or a set of sections, OR
b) allow a set of paragraphs to be provided before any sections of a release

Start by locating the changelog release parser and its tests. Check how a release body is currently required to contain sections, then support a no-change paragraph such as “There are no changes in this release” without requiring a pull-request association. Add coverage for the paragraph-only release and confirm that --pr-links remains usable.

Written by the indexing model from the issue text.