Add NullRedactor to the RedactorProvider during initialization

[makazeu]

Fixes #5265 - RedactorProvider returns ErasingRedactor for DataClassification.None type

Prior to this PR, the below unit test was failing because NullRedactor's not included in RedactorProvider's _classRedactors by default.

    [Fact]
    public void RedactorProvider_Returns_NullRedactor_For_NoneDataClassification()
    {
        var redactorProvider = new RedactorProvider(
            redactors: [ErasingRedactor.Instance],
            options: Microsoft.Extensions.Options.Options.Create(new RedactorProviderOptions()));

        Assert.IsType<NullRedactor>(redactorProvider.GetRedactor(DataClassification.None));
    }

This PR adds the NullRedactor to RedactorProvider when being initialized if the redactorMap doesn't contain DataClassification.None.

Microsoft Reviewers: Open in CodeFlow

[dariusclay]

I think this makes sense, just a minor nit