Code style: Fix ESLint "no-empty-function" errors

Part of #407.

lib/compiler/visitor.js

@@ -7,7 +7,9 @@ let visitor = {
       return functions[node.type].apply(null, arguments);
     }
 
-    function visitNop() { }
+    function visitNop() {
+      // Do nothing.
+    }
 
     function visitExpression(node) {
       let extraArgs = Array.prototype.slice.call(arguments, 1);