internal/contour: ingressroute/httpproxies outside --root-namespaces create empty route sets

[davecheney]

#2106 suggested that in some cases an ingressroute/httpproxy outside the root namespace set could cause empty records to be created in RDS.

This behaviour is unexpected. I would expect that empty ingress_http and ingress_https records are either always present or only present when there are valid routes. The current logic appears to be that those route entries will appear, but empty.

I suspect this is due to the late filtering that internal/dag.Builder does to filter out empty virtual hosts. The filtering is incomplete and internal/contour.RouteVisitor is spotting that there are virtualhosts without routes and causing the lds tables to be populated. I'll look into this next.

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