Parcelforce: Skip queries that start with ISBN.

Also added a check if `$parcel_number` exists in the first place.

lib/DDG/Goodie/Parcelforce.pm

@@ -34,6 +34,13 @@ triggers query_nowhitespace_nodash => qr/
 
 handle query_nowhitespace_nodash => sub {
     my $parcel_number = $+{parcel_number};
+   
+    # Exit if $parcel_number doesn't exist.
+    return unless $parcel_number;
+    
+    # Exit if $parcel_number starts with ISBN.
+    return if $parcel_number =~ /^isbn/i;
+    
     return $parcel_number,
         heading => 'Parcelforce Tracking',
         html =>