Hello K.,
unless, somewhere, anywhere, in the diagram, an arrow occurs.
But a Sequence diagram without arrow... is not a Sequence diagram...
Then here are some examples of Sequence diagram:
@startuml
' Add a participant in order to force to be a Sequence diagram
participant p
alt a
end
@enduml
@startuml
' Add a participant in order to force to be a Sequence diagram
participant p
group a
end
@enduml
Because this example is a Activity diagram:
@startuml
' Without participant this is a Activity diagram
'participant p
group a
end
@enduml
If that can help,
Regards,
Th.