Fix check for modified date

This check would produce an error if the modified date was NA. This was highlighted by the additional tests.

R/add_context.R

@@ -13,7 +13,7 @@ add_context <- function(data, id, name, created_date, modified_date) {
   # The platform can record the modified date as being before the created date
   # by a few microseconds, this will catch any rounding which ensure
   # created_date is always <= modified_date
-  if (modified_date < created_date) {
+  if (!is.na(modified_date) && modified_date < created_date) {
     modified_date <- created_date
   }