Support empty prefix

[roberth]

I'm trying to write a pkgs.moduleSystem library (for the option docs generating part, replacing nixosOptionsDoc), but passing that as a prefix results in rather long titles.
I figured --prefix "" would do the right thing, but that still generates a "separator", resulting in headers like .rendering, which isn't quite right.
The default prefix is lib, not an internal representation for "no prefix at all", unfortunately.

I propose that --prefix "" will be the accepted syntax for prefixing nothing at all; not even the separating ..

Context:

• doc/doc-support: Generalize library docs code NixOS/nixpkgs#305847

[infinisil]

Sounds reasonable to me, PR appreciated!