Add safeguard for VMs without ext_management_system

https://bugzilla.redhat.com/show_bug.cgi?id=1684649

app/services/network_topology_service.rb

@@ -71,7 +71,7 @@ def build_entity_data(entity)
     data[:display_kind] = entity_display_type(entity)
 
     if entity.kind_of?(Host) || entity.kind_of?(Vm)
-      data[:provider] = entity.ext_management_system.name
+      data[:provider] = entity.try(:ext_management_system).try(:name)
     end
 
     data