keep explicitly unconnected port bindings

These are now kept while still supporting optional trailing commas and
without introducing pass-through inconsistencies.

test/error/binding_mix_port_trail.sv

@@ -0,0 +1,9 @@
+// pattern: illegal mix of ordered and named port connections
+module example(
+    input a, b, c
+);
+endmodule
+module top;
+    wire a, b, c;
+    example e(.a(1), .b(2), ,);
+endmodule