Consider adding a way to represent empty strings

[Infernio]

We may need them for command defaults. Note that #19's fourth example used to have empty strings as defaults, but I had to change those since we don't accept them.

Another option would be to add support for passing empty strings as arguments, but then we run into the problem of parser ambiguities (which was why I dropped that in the first place).

So probably better to come up with some way to represent empty strings (at least for defaults). Maybe something like ∅ (see Null sign)? Users won't really have to ever enter it, so it being fancy Unicode stuff doesn't really hurt.