Add tests for null paths in navigation

Unfinished chapters/articles should be put in navigation

test/fixtures/SUMMARY.md

@@ -5,3 +5,6 @@
     * [Article 2](chapter-1/ARTICLE2.md)
 * [Chapter 2](chapter-2/README.md)
 * [Chapter 3](chapter-3/README.md)
+* [Chapter 4](chapter-4/README.md)
+    * Unfinished article
+* Unfinished Chapter

test/navigation.js

@@ -65,4 +65,10 @@ describe('Summary navigation', function() {
         assert.equal(nav['chapter-2/README.html'].level, '2');
         assert.equal(nav['chapter-3/README.html'].level, '3');
     });
+
+    it('should not accept null paths', function() {
+        var nav = navigation(LEXED);
+
+        assert(!nav[null]);
+    });
 });