bug#12351: 24.1; parse-colon-path turns empty paths into nil

[Glenn Morris]

Dave Abrahams wrote:

> Every OS I know of treats empty elements in "colon paths" (semicolon
> paths on Windows) differently from elements that are "."

So why do you want to just unconditionally delete them from the result
of parse-colon-path? What should they be represented by, if not nil?

load-path, exec-path both are documented to accept nil for
default-directory. cd-path works the same way.