Fix #55
Only drop the last empty component. The new behavior has the following effects:
1. The first empty component is preserved and can be treated specifically as
requested in #55. The assumption is that a separator at the beginning
is intentional.
(cdr (orderless-compile " ")) => ("")
(cdr (orderless-compile " foo")) => ("" "foo")
2. Completely blank input will compile to an empty regexp list. This is
important for completion command startup performance:
(cdr (orderless-compile "")) => nil
3. When separating components, the last separator will not lead to an
intermediate artificial empty component, as long as the user has
not finished typing. For example the user types the words "foo bar":
(cdr (orderless-compile "foo")) => ("foo")
(cdr (orderless-compile "foo ")) => ("foo")
(cdr (orderless-compile "foo bar")) => ("foo" "bar")