$ mongodump --uri $MONGO_PROD_READONLY
2020-05-11T19:15:05.085-0700 writing mydb.users to
2020-05-11T19:15:05.453-0700 writing mydb.posts to
2020-05-11T19:15:05.638-0700 done dumping mydb.posts (59 documents)
2020-05-11T19:15:05.805-0700 done dumping mydb.users (384 documents)

A space can be found after "to", which suggests mongodump tried to output an empty string.