cxxrtl: failing test: unconnected blackbox outputs don't compile.

tests/cxxrtl/test_unconnected_output.v

@@ -0,0 +1,24 @@
+(* cxxrtl_blackbox *)
+module blackbox(...);
+    (* cxxrtl_edge = "p" *)
+    input clk;
+
+    (* cxxrtl_sync *)
+    output [7:0] out1;
+
+    (* cxxrtl_sync *)
+    output [7:0] out2;
+endmodule
+
+module unconnected_output(
+    input  clk,
+           in,
+    output out
+);
+    blackbox bb (
+        .clock (clock),
+        .in    (in),
+        .out1  (out),
+        .out2  (/* unconnected */),
+    );
+endmodule