Fix SQLITE_ERROR when where:{} is used

Without this guard block, at times invalid SQL is generated by the module. Specifically if where:{} is specified in the query. I believe it is a fairly common use case to initialize a filter_condition with an empty object and then add conditions to it and pass it as where argument. Having no conditions is a special case, and when it can be dealt with at the module level trivially easily, it is best to do so.