FIX: phpBB3 importer created invalid quote for posts (#20646)

for proper quotes (those including a valid reference to a source post), the
importer failed to yield a username, and the imported quote was broken.

script/import_scripts/phpbb3/support/bbcode/xml_to_markdown.rb

@@ -203,7 +203,8 @@ def visit_QUOTE(xml_node, md_node)
     def quoted_post(xml_node)
       if @quoted_post_from_post_id
         post_id = to_i(xml_node.attr("post_id"))
-        @quoted_post_from_post_id.call(post_id) if post_id
+        username = quoted_username(xml_node)
+        @quoted_post_from_post_id.call(post_id).merge({ username: username }) if post_id && username
       end
     end