Support empty categories

src/invidious/helpers/extractors.cr

@@ -321,13 +321,15 @@ private module Parsers
         content_container = item_contents["contents"]
       end
 
-      raw_contents = content_container["items"].as_a
+      raw_contents = content_container["items"]?.try &.as_a
+      if !raw_contents.nil?
         raw_contents.each do |item|
           result = extract_item(item)
           if !result.nil?
             contents << result
           end
         end
+      end
 
       Category.new({
         title:            title,