roachtest/awsdms: fix race condition that can cause panics

In cockroachdb#95518, we added a new test case to check for a table error
during the DMS process. However there is a race when the check
happens depending on how quickly tasks have ran and the task
ReplicationTaskStats can be nil and we tried to access field
on the nil object that was returned from the API call.

This commit checks that the ReplicationTaskStat is not nil
before accessing the TablesErrored property.

Fixes: cockroachdb#93305
Release note: None

pkg/cmd/roachtest/tests/awsdms.go

@@ -327,9 +327,11 @@ func checkDMSNoPKTableError(ctx context.Context, t test.Test, dmsCli *dms.Client
 					}
 				}
 				for _, task := range dmsTasks.ReplicationTasks {
-					if task.ReplicationTaskStats.TablesErrored == 1 {
-						t.L().Printf("table error was found")
-						return nil
+					if task.ReplicationTaskStats != nil {
+						if task.ReplicationTaskStats.TablesErrored == 1 {
+							t.L().Printf("table error was found")
+							return nil
+						}
 					}
 				}
 				return errors.New("no table error found yet")