Simplify account/operation filtering implementation in sql_serializer

The following discussion from !89 (merged) should be addressed:

• @bwrona started a discussion:

  In my opinion using templates to specialize 'empty' implementation is overcomplicated. It also is slower solution than simple check at sql_serializer side (as filter class client) i.e basing on not-null main filter object. Now we need to call virtual method for it.