schemas/validate: Add EmptyOr validator
schemas/validate: Add EmptyOr validator

Harry-R/skylines,shadowoneau/skylines,RBE-Avionik/skylines,RBE-Avionik/skylines,Harry-R/skylines,skylines-project/skylines,shadowoneau/skylines,Harry-R/skylines,Turbo87/skylines,shadowoneau/skylines,Harry-R/skylines,skylines-project/skylines,Turbo87/skylines,Turbo87/skylines,skylines-project/skylines,shadowoneau/skylines,RBE-Avionik/skylines,RBE-Avionik/skylines,skylines-project/skylines,Turbo87/skylines