MixpanelPeople::set causes 10 calls if array of properties is empty.

[shaunsosw]

Would be better to have some error handling if an empty array is given of props instead of trying to make 10 curl calls to: https://api.mixpanel.com/engage each responding with curl error 28 (timeout).

Line 40 of lib/producers/MixpanelPeople.php