ServerVector addFeaturesInternal fails when features lack an id

`ol.source.ServerVector::addFeatures` will only add the first of an array of features if the features lack an `id` property because `addFeaturesInternal` tries to track them by id to avoid loading the same feature twice.  But if the features return nothing for `getId()` then only the first feature will be loaded.
What if featureId equals 0? You probably want to use `goog.isDef` here?