Cannot use anonymous credentials when overriding a mail configuration in a subwiki

Discussed on forum

There seems to be bug in the overriding subwiki mail config with main wiki settings if subwiki mail config is not/partially set.

Situation:

1. mainwiki mail settings - SmtpServer1 with username1/password1
2. subwiki mail settings - SmtpServer2 with anonymous access (username/password fields empty)

Result:
Subwiki is trying to send mail using SmtpServer2 with username1/password1 resulting in sending error

Expected result:
Subwiki is using SmtpServer2 with anonymous access

(In other words if smtp server on subwiki is set, then always use username/pwd from subwiki)