Network Admin Menu: 

* Don't display update count for Themes when the user doesn't have the 'update_themes' cap
* Don't display update count for Plugins when there aren't any/count is `0`

Props peterchester.
Fixes #29651.


git-svn-id: https://develop.svn.wordpress.org/trunk@29817 602fd350-edb4-49c9-b593-d223f7449a82