Abandon plans to change viacode's return of unassigned

The BUGS section of the charnames pod said that it was a bug to return
undef for unassigned characters, whereas the real Unicode name is the
empty string.  demerphq noted that undef stringifies to the empty
string, so we are in fact in compliance with the standard.  This
clarifies the pod wording, removing the text from the BUGS section.