Allow READ-FROM-MINIBUFFER-IN-EMACS to return an empty string

[daewok]

The docstring for READ-FROM-MINIBUFFER-IN-EMACS says that an empty string is
returned when a user enters nothing. Prior to this commit,
SLY-READ-FROM-MINIBUFFER was called in such a way that an empty result wasn't
allowed.

[daewok]

It kind of looks like the t that was already present in the call to sly-read-from-minibuffer may have been intended for the allow-empty optional argument, but instead it's consumed by the hist optional argument.

So I'm unsure if the correct fix is (sly-read-from-minibuffer prompt initial-value t t) (history is not saved) or (sly-read-from-minibuffer prompt initial-value nil t) (history is saved).