Fix avatar bug, add a6

cards_a6_lettersheet.typ

@@ -0,0 +1,12 @@
+#{
+  set page("us-letter", margin: 0em)
+
+  import "common.typ"
+
+  let options = json("options.json")
+  let cards = options.cards
+  let args = options.args
+
+  common.card_sheets(148mm, 105mm, 1in/16, args, cards)
+
+}

common.typ

@@ -49,7 +49,7 @@
       )
     )
   }
-  if card.avatar != "" {
+  if card.avatar != none {
     place_avatar(text_height, card)
   }
 }