Is this change right?  The old way, if `GI::Deurl::XS::parse_query_string($source)` returned undef, `$self->{_query_params}` would get set to an empty hashref; with the change, it won't be set to anything.

I haven't looked closely enough to see if this change will cause breakage elsewhere, but if simply changing for the sake of avoiding the defined-or operator, I'd like to see it keep the previous behaviour.

The docs for `CGI::Deurl::XS` suggest it will return a hashref or undef, so just substituting `//` with `||` should be sufficient, I think.