[BUGFIX] SolrService checks synonyms for wrong type

It checked for an array, but json_decode provides a stdClass.
In test it worked, because the fallback to default language matched.

Resolves: #60432
Change-Id: Ia6853902c6af5167a58231825d5e0cb12bfe1b00
Reviewed-on: http://review.typo3.org/32105
Reviewed-by: Hans Höchtl <jhoechtl@gmail.com>
Tested-by: Hans Höchtl <jhoechtl@gmail.com>