IronMqProvider no message bug fixed

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

IronMq client returns a null value when no messages found in queue therefore the following foreach loop throws an invalid argument error.

Here you can see the line at which it returns a null value: 
https://github.com/iron-io/iron_mq_php/blob/master/IronMQ.class.php#L334