www: do not warn on blank query parameters

www: do not warn on blank query parameters

Sometimes users (or bots) may lead queries with '&' and
trigger uninitialized variable warnings, just ignore them
and give consumers a $ctx->{qp}->{''} entry.

While we're in the area, pass a regexp rather than scalar string
to the `split' perlop to prevent Perl from recompiling the
regexp on every call.