Fix undefined hash reference warning

This fixes a bug introduced in commit
5e3e10d32e

For some reason Perl is OK with:

    my $x = undef;
    my @k = keys(%$x);  # empty list, no warnings

but not:

    my $x = undef;
    my %h = %$x;
    my @k = keys(%h);

ddclient.in

@@ -1950,7 +1950,7 @@ sub init_config {
         ## merge options into host definitions or globals
         if (@hosts) {
             for my $h (@hosts) {
-                $config{$h} = {%{$config{$h}}, %options};
+                $config{$h} = {%{$config{$h} // {}}, %options};
             }
             $opt{'host'} = join(',', @hosts);
         } else {