Make the PerSystSqlOperator less verbose

analytics/operators/persystsql/PerSystSqlOperator.cpp

@@ -172,7 +172,7 @@ void PerSystSqlOperator::compute(U_Ptr unit, qeJobData& jobData) {
 	}
 	uint64_t measurement_ts = 0;
 	if(_buffer.size() == 0){
-		LOG(error) << "PerSystSql Operator " << _name << ": no data in queryEngine found!";
+		LOG(debug) << "PerSystSql Operator " << _name << ": no data in queryEngine found!";
 		return;
 	} else {
 		measurement_ts = _buffer[0].timestamp;