support nameless/noteless releases

[JeanMertz]

The current template expects there to be an annotated tag and will use the first line of the annotation as the release name.

There are two extra cases that should be supported I think:

1. A release with release notes, but without a release name.
2. A release without a name or release notes.

I think the following could work:

• If there is an annotated tag, it has a non-empty first line, and no second line, then it's a named release without notes.
• If there is an annotated tag, it has a non-empty first line, an an empty second line, and a non-empty third line, then it's a named release with notes.
• If there is an annotated tag, with a non-empty first line and a non-empty second line, then it is an unnamed release with release notes.
• If there is no annotated tag (i.e. a lightweight tag), then it is an unnamed release without release notes.

This isn't airtight, since someone could be writing release notes on a single line without line breaks, and it would end up as a release name instead. We could do some shenanigans of checking the length of the first line, and anything above 50 is considered to be an unnamed release with notes.

[JeanMertz]

Partially resolved in 6b79f87. The only thing left is to support unnamed release with release notes.