Add a fail test case (#2712)

test/unit/specs/parsers/expression_spec.js

@@ -218,6 +218,12 @@ var testCases = [
     expected: true,
     paths: []
   },
+  {
+    exp: 'null',
+    scope: {},
+    expected: null,
+    paths: []
+  },
   {
     // Date global
     exp: 'Date.now() > new Date("2000-01-01")',