Change error wording for \o{}

Change error wording for \o{}

An empty \o{} no longer says "Number with no digits" in favor of "Empty
\o{}" which is more consistent with errors raised for things like \b{},
\P{}.

There is a small risk of breakage with this change, as with any
diagnostic wording change.  However, this construct is relatively new
and rarely used, and this is a fatal error, not a warning you might want
to trap on.  There are no empty \o{} instances in CPAN.