Hi Angarato,
Your query could look like this:
FIND MyObject WHERE (MyObject.Attr1 CONTAINS SearchParams.Param1 OR SearchParams.Param1 IS UNDEFINED) AND (MyObject.Attr2 CONTAINS SearchParams.Param2 OR SearchParams.Param2 IS UNDEFINED) AND ...
Any of the Param attributes in SearchParams object left blank by the user will be effectively excluded from the filtering.