Filter out empty sections when generating pages

lib/parse/page.js

@@ -55,6 +55,9 @@ function parsePage(src) {
         section.type = sectionType(section, idx);
         return section;
     })
+    .filter(function(section) {
+        return !_.isEmpty(section);
+    })
     .map(function(section) {
         // Generate a uniqueId to identify this section in our code
         var id = _.uniqueId('gitbook_');