Use uuid.Nil instead of uuid.New() in ent schema initializers

It is used just to carry the sql.Scanner and sql.Valuer interface.

pkg/model/ent/schema/session.go

@@ -13,7 +13,7 @@ type Session struct {
 
 func (Session) Fields() []ent.Field {
 	return []ent.Field{
-		field.UUID("id", uuid.New()).Immutable(),
+		field.UUID("id", uuid.Nil).Immutable(),
 		field.Time("created").Immutable(),
 		field.Time("started").Nillable().Optional(), // TODO remove?
 		field.String("code").MinLen(6).MaxLen(6).Immutable().Unique(),