Allow empty releases

[Currently](https://github.com/MetaMask/auto-changelog/blob/1c931ec77867f2d70ff5197cc25ee7dba99d2425/src/update-changelog.ts#L177), 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.

Start at src/update-changelog.ts around line 177, then trace how the --rc option is handled. Add support for --allow-empty-releases so an RC with no commits can add a no-changes section, while other cases still return undefined; verify both behaviors with the repository's existing checks.

Written by the indexing model from the issue text.