FIX: Include previously solved topics in auto-bump

Null values can happen when a post is removed as the solution. We already filter for NOT NULL in the topic_query and search filters. This commit adds the same logic to the auto-bump filter.

plugin.rb

@@ -545,6 +545,7 @@ SQL
           SELECT 1 FROM topic_custom_fields
           WHERE topic_id = topics.id
           AND name = 'accepted_answer_post_id'
+          AND value IS NOT NULL
         )
       SQL
     })