Aggregate empty filter too broad and may inappropriately filter results

This is a regression from TEIID-2253 which consolidated logic in RulePushAggregates to handle multi-source planning. It also inappropriately expanded an empty test (which should only be applied to grouping without a group by) to some cases with group by clauses and in an attempt to broaden pushdown support utilized is not null tests. However the is not null tests are not generally valid as the aggregate could be computed against all null values for example. Depending upon this situations this could result in invalid filtering of results.