[BUGFIX] SolrService::getSynonyms should always return an array

There is an exception thrown when the Synonym backend module is
opened but no synonyms exist in the solr core.

This is caused by the function SolrService::getSynonyms that
should always return an array which is ensured here.

Change-Id: Idda688e1b0d079a746d172268b19c891063d36cd
Resolves: #60432
Reviewed-on: http://review.typo3.org/31701
Reviewed-by: Hans Höchtl <jhoechtl@gmail.com>
Tested-by: Hans Höchtl <jhoechtl@gmail.com>