Down sampling fails when target & comparator have 0 records

[anthonysena]

In CohortMethod v6, this code fails when the T&C cohorts have 0 people resulting in an attrition with 0 entries.

https://github.com/OHDSI/CohortMethod/blob/e09adce65a256b613ea581cd627cd9cfc7027d10/R/DataLoadingSaving.R#L299-307

The error raised is:

Error in if (preSampleCounts$targetExposures > maxCohortSize) { : 
  argument is of length zero