Fall back to blank image if alt style has no preview image

This is currently crashing the Rainbow Pool when the Anniversary Techo
would appear, because the asset seems to be missing? The SWF doesn't
seem to exist, nor does its manifest.

app/models/alt_style.rb

@@ -62,11 +62,10 @@ class AltStyle < ApplicationRecord
     "#{series_name} #{name}"
   end
 
+  EMPTY_IMAGE_URL = "data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
   def preview_image_url
-    swf_asset = swf_assets.first
-    return nil if swf_asset.nil?
-
-    swf_asset.image_url
+    # Use the image URL for the first asset. Or, fall back to an empty image.
+    swf_assets.first&.image_url || EMPTY_IMAGE_URL
   end
 
   # Given a list of items, return how they look on this alt style.