Bug 19389: Hide library groups pull down if empty

Bug 19389: Hide library groups pull down if empty

Regression introduced by bug
  commit 141200794da9f27e0231e1c2dad4bcb994d650f3
    Bug 15295: Koha::Libraries - Remove GetBranchCategories

The intranet advanced search page offers to search for groups of
libraries, even if the pull down is empty as no library groups have
been defined.

Test plan:
- Go to the adv search page at the intranet
- Without library group you must not see the "Groups of libraries"
dropdown list
- With at least a library group you must see it

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>