Assert `null` value render

It was not being asserted anywhere else, and I was suspecting that
PR #552 was changing current (and correct) behaviour.

test/_files/escaped.js

@@ -2,5 +2,6 @@
   title: function () {
     return "Bear > Shark";
   },
+  symbol: null,
   entities: "&quot; \"'<>`=/"
 })

test/_files/escaped.mustache

@@ -1,2 +1,2 @@
-<h1>{{title}}</h1>
+<h1>{{title}}{{symbol}}</h1>
 And even {{entities}}, but not {{{entities}}}.

test/_files/unescaped.js

@@ -1,5 +1,6 @@
 ({
   title: function () {
     return "Bear > Shark";
-  }
+  },
+  symbol: null
 })

test/_files/unescaped.mustache

@@ -1 +1 @@
-<h1>{{{title}}}</h1>
+<h1>{{{title}}}{{{symbol}}}</h1>