@Dvorapa: edit No, I checked the empty value today. The `db_username`/password using ONLY if `= None`. Take a look to code, lines 57-65. And you can see in debuger - there on value '' or empty using `{'read_default_file': config.db_connect_file}`.
Topic on Manual talk:Pywikibot/MySQL
Yeah, but in Python at least: '' == None too. And also if no value ever set, Pywikibot really uses the default from config2, which is also ''. So it doesn't matter if db_connect_file is '', None or not set in the user-config, still db_username counts in all these cases. Therefore better not push users to change '' to None (as they are equal) and leave it as is.
Sorry, but you mistake. 1) '' == None is False, see online console. 2) Users have error on connect to database, if do not set `db_connect_file = None`. Screenshot.
I can not reproduce what you describe, maybe you should fill in a bug task to Phabricator as the behavior in this manual should be described correctly.