FIX: Import posts of missing users from phpbb3 (#9085)

Posts without a user probably shouldn't happen unless there was some direct database tampering, but data like that has been seen in the wild.

The importer will assign those posts to the "system" user.

script/import_scripts/phpbb3/database/database_3_0.rb

@@ -83,7 +83,7 @@ module ImportScripts::PhpBB3
           CASE WHEN u.user_type = #{Constants::USER_TYPE_IGNORE} THEN p.post_username ELSE NULL END post_username
         FROM #{@table_prefix}posts p
           JOIN #{@table_prefix}topics t ON (p.topic_id = t.topic_id)
-          JOIN #{@table_prefix}users u ON (p.poster_id = u.user_id)
+          LEFT OUTER JOIN #{@table_prefix}users u ON (p.poster_id = u.user_id)
         WHERE p.post_id > #{last_post_id}
         ORDER BY p.post_id
         LIMIT #{@batch_size}