Previously in Nix commands like log always required an argument. This was confusing, because it is often used in combination with nix build, but does not expect the same arguments:

$ nix build
$ nix log
error: more arguments are required
Try 'nix --help' for more information.

A workaround was to use "" as argument, since that same value is used by default for the build command:

$ nix build
$ nix log ""

This is because nix log has the following usage:

Usage: nix log <FLAGS>... <INSTALLABLE>

whereas nix build has:

Usage: nix build <FLAGS>... <INSTALLABLES>...

This PR will make the INSTALLABLE of nix log, nix eval and nix edit optional. That makes the usage as follows:

Usage: nix log <FLAGS>... <INSTALLABLE>?

Solves #2332 #2066
Partially solves #2078