fix TypeMismatchError in Chrome Stable

fixes TypeMismatchError: The type of an object was incompatible with the
expected type of the parameter associated to the object. when turnServer
is empty.

Reproduces in Chrome Stable:

new webkitRTCPeerConnection({iceServers: [{url: 'stun:stun.l.google.com:19302'}, {credential: "", username: "", url: ""}]}, {optional: [{DtlsSrtpKeyAgreement: true}]}); will fail with the error.