Re: file-readable-p returns t for empty string

[Richard Stallman]

      So what is expand-file-name to do with a
    syntactically [invalid] name?  I see three possibilities:

    a) just leave it alone.  Someone else will have to deal with it.
    b) return NIL
    c) signal an error

    So I'd think a) the most expedient.

In this context, I am not sure what "just leave it alone" means.
Do you mean, return ""?  That would violate the principle that
expand-file-name always returns an absolute file name.
So it would not be a good idea.

Returning nil would break lots of C code that assumes expand-file-name
always returns a string.

Perhaps signalling an error is the best option.