Fix bug with return value in abstract reference filter

When `object_class.link_reference_pattern` was `nil` because object
being processed do not implemement link references `call` method
returned `nil` instead of returning document, even if document has been
processed in for basic reference pattern.

lib/banzai/filter/abstract_reference_filter.rb

@@ -94,6 +94,8 @@ module Banzai
             object_link_filter(link, object_class.link_reference_pattern, link_text: text)
           end
         end
+
+        doc
       end
 
       # Replace references (like `!123` for merge requests) in text with links