Fix empty auto-summaries triggering a fatal error.

Fix empty auto-summaries triggering a fatal error.

Aka:  Streamline Linker::formatAutocomments() and add tests

This uses the "streamlining" for the code proposed by Thiemo
in I38edc1ad7720. I have squashed the two commits, so it now
has his code in Linker, but still has my tests as well as his.

Thiemo wrote on his patch:
This also changes the output in case there is no fragment to link to.
Before an empty `/* */` in a summary this would have created a link to
the page. I would like to argue this is not what a user expects.

Bug: T222628
Change-Id: I05408ede0e20dfd976f4057fc5baab461d2ef769