Provide a default DataSource username if none has been provided

[snicoll]

Fixing #23538 detects actual embedded use cases. The side effect is that we don't set a username anymore with, for instance, file-based H2 persistence, See #23721 (comment)

This leads to a regression as H2 will create the database with whatever credentials were provided initially. With a default setup Spring Boot will configure a sa user with no password. By upgrading, it would have no user and no password. We should revisit this to make this a bit more consistent while allowing users to specify an empty username if they want to.

[snicoll]

We've discussed and decided to keep things as they are. It is a breaking change but the previous behaviour of setting a sa username for a non embedded database was wrong.

I've added a noteworthy note in the related issue #23693 (comment).