Move NullLoggerFactory and NullLoggerOfT to Microsoft.Extensions.Logging.Abstractions

[ajaybhargavb]

From aspnet/DataProtection#203 (comment)

We currently have NullLogger and NullLoggerProvider in Logging.Abstractions but NullLoggerFactory is in Logging.Testing. It should be moved to Logging.Abstractions so it can be used in other places like the one referenced above.

@Eilon

[Eilon]

@muratg @davidfowl @DamianEdwards - any objections? This seems like good cleanup because we have a mess of NullLoggerXyz's scattered around.

[davidfowl]

👍