Issue with Rubicon adapter in case of no ad

[jdelhommeau]

Sometime Rubicon fastlane code send back a response stating no ads available.
This is not well interpreted by prebid.js which is looking for an ad and fail on the line 888:

                   var sizeArray = size.dim.split('x');

size is undefined, which cause an error:

Error parsing rubicon response bid: 'undefined' is not an object (evaluating 'size.dim')

This is the no ad response from Rubicon which is mis interpreted.

[hjeong12]

fixed in #130