SpringBoot Databases that support embedded and non-embedded modes are always detected as embedded

backport of #23693 in 2.3.x.

Comment From: snicoll

By switching from sa to null for non-embedded use cases, any file-based database that was created prior to upgrading will require users to set the username (spring.datasource.username=sa).

I don't think the related problem (being able to user a file-based database without a username is worth breaking this use case so I am going to revert the backport.