0005995: search with spaces only causes MySQL exception

This fix trims the `searchparam` parameter before checking conditions indicating empty searches.
For further information see https://bugs.oxid-esales.com/view.php?id=5995

Space-only values for the other search related parameters (`searchcnid`, `searchvendor`, `searchmanufacturer`) don't cause the same effect (MySQL exception).

The root cause for this bug lies in `oxSearch::_getWhere()` and may need further treatment. But because OXID eShop should handle space-only searchparam values like empty values and because of my assumption that the other call traces to this method don't involve user input I implemented the trim-call in the search controller and added an unit test for this case.