Fixed palette model to be a Struct

Necessary since some keys might be missing so this'll ensure those keys default to `nil`.

Milestone: patch

lib/trmnl/api/models/palette.rb

@@ -4,7 +4,7 @@ module TRMNL
   module API
     module Models
       # Models the data of the API response.
-      Palette = Data.define :id, :name, :grays, :colors, :framework_class do
+      Palette = Struct.new :id, :name, :grays, :colors, :framework_class do
         def self.for(attributes) = new(**attributes)
       end
     end