Stop scaffolding DbSet initialization to null!

[roji]

Now that we have a diagnostics suppressor for uninitialized DbSet properties (#26795), we can stop scaffolding DbContext code which initializes DbSet properties to null!.

[roji]

After #27869 we should properly suppress all uninitialized DbSet warnings.

However, for cases where a constructor exists on the DbContext, our new suppressor depends on a new Roslyn enhancement added in 7.0.0-preview3 (dotnet/roslyn#58073). This means that when using an older SDK, those specific warnings would not be suppressed.

This isn't a big problem in itself (not worse than today), but could be an issue if we change scaffolding to remove the bang initialization (this issue); if EF Core 7.0 will target .NET 6.0 as it does today, it would be usable with SDK 6.0 as well, where the scaffolded code would start generate a warning.

[ajcvickers]

Note from triage: looks like an issue with analyzers during tests. @roji will investigate.

[roji]

@bricelam I did some trivial testing from a console program against 7.0.0-preview.6, and everything seems to be working correctly - I'm getting no uninitialized warnings for DbSets (with a DbContext, constructor or without it). So I guess there's some testing issue there - if you want to point me to some failing test I can look...