Fix #187 - Setting ice_servers to [] does not disable ICE

[jirkah]

Hello, just a simple PR:

• Fixes https://github.com/DoubangoTelecom/sipml5/issues/187,
• based on the proposition in the issue, credits to the author,
• important to use with Chrome v57 and without ICE.

Please let me know if something needs to be modified.

Many thanks, Jirka