MPIDE-27: remove undefined outputs

Parser was resulting in some fields having undefined values, causing
errors when reading it as JSON.

src/lib/parser/modellang.pegjs

@@ -181,8 +181,8 @@
             ast.Base.call(this, "Selector");
             this.system = system;
             this.pattern = pattern;
-            this.pre = pre;
-            this.post = post;
+            this.pre = pre || null;
+            this.post = post || null;
         },
 
         Trigger: function Trigger(selector, body) {

src/lib/parser/test/parser.js

@@ -1,15 +1,15 @@
 "use strict";
 
 let assert = require("assert");
+
 require("../../../../build/lib/parser/modellang");
 let Parser = modellangParser;
 
-
 let parseOpts = {locations: false};
 
 describe("Parser", function() {
 
-	describe("#parse", function() {
+	describe("#parse()", function() {
 
 		it("should parse basic sequences", function() {
 			let input = "SYSTEM: a = b c ((d e) f);\n";