Does not work with `--no-increment` flag

[ofhouse]

Hi, I tried to use the plugin in a CI system together with the --no-increment from the release-it CLI.

My workflow:

1. Initial Changelog

   # Changelog
   
   ## [Unreleased]
   - Some changes

2. Generate the release locally

   # Changelog
   
   ## [Unreleased]
   
   ## [v1.0.0]
   - Some changes

3. Push the changes to GitHub
4. Publish the release from CI

   release-it --no-increment --ci

Expected behavior

I would expect that the plugin does not try to update the changelog, when the --no-increment flag is set.

Actual behavior

The plugin throws an error when the "Unreleased" section is empty:

ERROR There are no entries under "Unreleased" section in CHANGELOG.md.