"no-empty-group" should not trigger for `unreleased`

[koppor]

Follow-up to #406

When creating an unreleased section, the common practice is to have empty headings to guide contributors where to put things.

Maybe, the rule could ignore Unreleased?

    26:1  error  Heading [Unreleased] has no entries for REMOVED
                                    no-empty-group

---

In my context, I think, I will just disable this rule.