Don't fail when no cards

This will prevent nil errors when parsing the command. Instead, the regular command validation will trigger and the error will be handled as expected.

app/models/command/cards.rb

@@ -4,7 +4,7 @@ module Command::Cards
   included do
     store_accessor :data, :card_ids
 
-    validates_presence_of :card_ids
+    validates_presence_of :card_ids, :cards
   end
 
   def undoable?

app/models/command/parser/context.rb

@@ -11,6 +11,8 @@ class Command::Parser::Context
       user.accessible_cards.where id: params[:id]
     elsif controller == "cards" && action == "index"
       filter.cards
+    else
+      Card.none
     end
   end