Beachball doesn't allow empty change comments

[Paulius Valiūnas]

We currently use Rush but are investigating options to migrate to pnpm workspaces and Lage instead. Beachball feels similar to Rush but there's one major difference: it doesn't allow supplying a blank message to the change file. This is an interesting change because that's the default behavior of Rush when you don't provide your own message. I think it would make sense to support empty messages because not all changes (especially bumping some internal dependency) deserve their own entry in the changelog.