It would be easier if the system when prompting for the Master Password, had a "I don't want to define a Master Password" or something like that, which would set that config_local.py property automatically.
We very intentionally don't do that as 1) allowing pgAdmin to rewrite it's own application configuration would create a security hole, and b) it would prevent sysadmins from being able to enforce a security policy on their users in managed environments.