[#30925]: Hide 'plus' icon for empty subcategories

Updated PR against the "staging" branch.

The 'plus' button for subcategories should only be displayed if there are subcategories that can be extended. 

JoomlaCode item: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30925
Old PR: #1699