Release tool: provide an indication when CHANGELOG entry has no content

One of the steps of the release process is CHANGELOG entry generation. Sometimes the entry might have no content (for example when only docs/refactoring changes were merged since the previous release) and the entry looks like this:

```

# 13.0.2 (2021-11-17)
## Special Thanks
Andrew Kushnir, Armen Vardanyan, Dylan Hunn, Joey Perrott, Martin von Gagern, Paul Gschwendtner, Pete Bacon Darwin, Ramesh Thiruchelvam, dario-piotrowicz and fusho-takahashi

```

Release instructions used to suggest that Caretakers should add the following phrase in this case:

```
This release contains various API docs improvements.
```

The proposal is to warn a Caretaker that actions need to be taken to update the CHANGELOG.

P.S.: issue #263 also results in adding the same update to the PR generated against the master branch during the patch release.

Beginne damit, den Schritt zur Generierung des CHANGELOG-Eintrags des Release-Tools zu lokalisieren, und reproduziere ein Release, dessen Eintrag nur die Überschrift und den besonderen Dank enthält. Bestätige, dass die Warnung den Caretaker auffordert, den CHANGELOG zu aktualisieren, einschließlich des gegen den master branch generierten patch-release PR; als erledigt gilt dies, wenn beide Fälle diesen Hinweis enthalten.

Vom Indexierungsmodell aus dem Issue-Text verfasst.