Stereotypes of empty components are not shown in diagrams...
The following patch should correct this :
https://filex.univ-jfc.fr/get?k=hfwiQLpaVIMzD7wjzYB
@startuml
rectangle "empty rectangle" <<foobar>> {
}
rectangle "outer" <<foobar>> {
rectangle inner
component "empty component" <<foobar>> {
}
@enduml