Fix table view that doesn't have private keys

[kurotych]

~~use object keyword for such~~

UPD: hide empty members should fix the problem + remove '---' from template if no PKs

https://github.com/plantuml/plantuml/issues/1304

STR:

@startuml

hide circle
skinparam linetype ortho

!include https://raw.githubusercontent.com/kurotych/sqlant/b2e5db9ed8659f281208a687a344b34ff38129cd/puml-lib/db_ent.puml

table(customer) {
  ---
  column(name, "text", $nn=true)
}

@enduml