The feature with undefined id is a multi-polygon representing all countries. Instead of rendering all multi-polygons with a lower z-index, we just avoid rendering this single feature.
When a style has no rules, the "else" symbolizers apply. When a style has rules and none of them apply to the given feature, the "else" symbolizers apply. Note that this is different than default symbolizer properties that might be merged into all symbolizers (as in OL2) - I don't think we should support that.