chore(derp): allow nil regions in DERPMaps

Previously, nil regions were not properly handled. This change allows users to disable regions in DERPMaps.

Particularly useful to disable some official regions.

hscontrol/derp/derp.go

@@ -82,6 +82,12 @@ func mergeDERPMaps(derpMaps []*tailcfg.DERPMap) *tailcfg.DERPMap {
 		maps.Copy(result.Regions, derpMap.Regions)
 	}
 
+	for id, region := range result.Regions {
+		if region == nil {
+			delete(result.Regions, id)
+		}
+	}
+
 	return &result
 }