Adding support for rendering empty subexpressions

Fixes #1

spec/parser/javascript/match_spec.js

@@ -52,6 +52,12 @@ describe('parser/javascript/match.js', function() {
       ],
       proxy: undefined
     },
+    '': {
+      anchorStart: false,
+      anchorEnd: false,
+      parts: [],
+      proxy: undefined
+    }
   }, (content, str) => {
     it(`parses "${str}" as a Match`, function() {
       var parser = new javascript.Parser(str);

src/js/parser/javascript/match.js

@@ -23,7 +23,8 @@ export default {
 
   _render() {
     var start, end,
-        partPromises;
+        partPromises,
+        items;
 
     if (this.anchorStart) {
       start = this.renderLabel('Start of line')
@@ -39,7 +40,13 @@ export default {
       return part.render(this.container.group());
     });
 
-    return Q.all(_([start, partPromises, end]).flatten().compact().value())
+    items = _([start, partPromises, end]).flatten().compact().value();
+
+    if (items.length === 0) {
+      items = [this.container.group()];
+    }
+
+    return Q.all(items)
       .then(items => {
         this.start = _.first(items);
         this.end = _.last(items);