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/service/hover.js b/src/service/hover.js
index a7cef71a..fca18130 100644
--- a/src/service/hover.js
+++ b/src/service/hover.js
@@ -50,7 +50,7 @@ angularServiceInject("$hover", function(browser, document) {
       }
     } else if (tooltip) {
       tooltip.callout.remove();
-      tooltip = _null;
+      tooltip = null;
     }
   });
 }, ['$browser', '$document'], true);