TRUNK-4922 ConceptDAO filters concept classes only if results were found
The skipSame-method in LuceneQuery generates a new instance of
fullTextQuery even if there are no search results. In this way all
concepts will be returned without filtering them.
* only call buildQuery() in skipSame when results were found
* add test to ConceptServiceTest
(cherry picked from commit 71503dbdf6e572d69efd2a0737e6075f0ea0063c)