Allow empty releases

[mcmire]

Currently, if we can't find any commits to add to the changelog, and the --rc option was provided or if there are already unreleased changes present in the changelog, the changelog is left unaltered and updateChangelog returns undefined.

This should be changed slightly:

• If the --rc option was provided and a new option --allow-empty-releases is provided, then a new section should be added to the changelog which merely reads something like "There are no changes for this release." (This is designed for monorepos which use a fixed versioning scheme, where even if a package has no changes, we may still want to create a new release for that package.)
• Otherwise, undefined should be returned.