Remove empty changelog sections when creating changelog from Keep A Changelog format

[weierophinney]

Feature Request

Q	A
New Feature	improvement
RFC	no
BC Break	no

Summary

Currently when using Keep A Changelog-formatted changelogs, it's not uncommon to have entries that look like this:

## X.Y.Z - TBD

### Added

- Nothing.

### Changed

- Nothing.

### Removed

- Nothing.

### Deprecated

- Nothing.

### Fixed

- Something we fixed

This leads to a lot of noise in the generated changelogs, as we have a lot of "Nothing" entries, followed by a list of things we actually did.

The suggested improvement is to find ### {heading}\n\nNothing.\n entries and remove them as part of changelog fetching and normalization.