Accept null connection string in UseMySql

[victor-borges]

As part of EF Core 7 there are migrations tooling improvements, including allowing null for UseSqlServer. It is not clear to me if that change is only during design time or if it was relaxed in every case, but that would be a nice feature for the MySQL provider for getting rid of "dummy" connection strings that are there just to instanciate the DbContext in design time (something I have to do every time when developing a multi-tenant app that doesn't have a default connection, might also just be my lack of knowledge of alternative methods).

[mguinness]

Appears related to the upstream issue dotnet/efcore#26869.