must be empty (#666)
must be empty (#666)

* must be empty

but not undefined

* - default switch case is not enough
- @tobiasschweizer prefers an empty string to a word "empty"

dhlab-basel/Knora,musicEnfanthen/Knora,dhlab-basel/Knora,dhlab-basel/Knora,musicEnfanthen/Knora,musicEnfanthen/Knora,dhlab-basel/Knora,dhlab-basel/Knora,musicEnfanthen/Knora,dhlab-basel/Knora,dhlab-basel/Knora,musicEnfanthen/Knora,musicEnfanthen/Knora,musicEnfanthen/Knora