improve error handling when no data exists in cdm

[fdefalco]

exportAO will fail with the following error message if data only exists in the person table, but no other tables:

Error in aggregate.data.frame(as.data.frame(x), ...) : no rows to aggregate

This error is occurring in the check for total records (https://github.com/OHDSI/Achilles/blob/main/R/exportAO.R#L1223) This should be modified to check for records returned from the query, if no records are returned provide an error message describing the problem and exit.