remove _null and _undefined

they have no significant effect on minified and gziped size. in fact
they make things worse.

file        | before     | after removal
----------------------------------------
concat      | 325415     | 325297
min         | 62070      | 62161
min + gzip  | 25187      | 25176

The bottom line is that we are getting 0.05% decrease in size after
gzip without all of the hassle of using underscores everywhere.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/Compiler.js b/src/Compiler.js
index 09857ea4..a03d24c4 100644
--- a/src/Compiler.js
+++ b/src/Compiler.js
@@ -330,7 +330,7 @@ Compiler.prototype = {
         template.addChild(i, self.templatize(child, i, priority));
       });
     }
-    return template.empty() ? _null : template;
+    return template.empty() ? null : template;
   }
 };