Javascript: support null pointers.

We allow to set pointer types using JS null.

Examples/test-suite/javascript/null_pointer_runme.js

@@ -0,0 +1,9 @@
+var null_pointer = require("null_pointer");
+
+if (!null_pointer.func(null)) {
+  throw new Error("Javascript 'null' should be converted into NULL.");
+}
+
+if (null_pointer.getnull() != null) {
+  throw new Error("NULL should be converted into Javascript 'null'.");
+}