core: simplify changeCaseDisplay method in Search

Simplify changeCaseDisplay method in search autocompleter.
Part of the expression was processing an empty string which has no
purpose.
Added one more test case to the existing unit test.

Change-Id: Ib2bc968c5fc724c64fc45804dd847c29994b7851
Signed-off-by: Alissa Bonas <abonas@redhat.com>