use nil instead of empty string

1 job for pr-upstream-http-proxy in 2 minutes and 10 seconds (queued for 7 seconds)
latest
Status Job ID Name Coverage
  Test
failed #1099
unit-testing

00:02:10

 
Name Stage Failure
failed
unit-testing Test There is an unknown failure, please try again
        (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3

15:08:41.563 [error] GenServer #PID<0.5629.0> terminating
** (DBConnection.ConnectionError) tcp connect (localhost:5432): connection refused - :econnrefused
(db_connection) lib/db_connection/connection.ex:148: DBConnection.Connection.connect/2
(connection) lib/connection.ex:622: Connection.enter_connect/5
(stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Last message: nil
ERROR: Job failed: exit code 1