Expose RelationalDatabaseCreator.HasTables

[ajcvickers]

I have been updating the DatabaseErrorPage tests in ASP.NET to use SQLite so as to be more reliable. I found that the tests failed because SQLite easily gets to a state where the database exists but is empty--for example, just opening a connection to a non-existent database can result it it now existing, but empty.

This can be fixed by checking if the database exists but has no tables, like we do for EnsureCreated, but to do that in a provider agnostic way requires the API to be exposed.