Fixed nulls for Success result

[lkurzyniec]

Collections should never return null. Better value for collection is just empty collection.
As a reference, you can take API - collection endpoints never return null, but empty array ([]).

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Null cause problems, ie I can't do like this Assert.True(result.IsSuccessful, $"Failing Types: {string.Join("; ", result.FailingTypeNames)}");, I need Assert.True(result.IsSuccessful, $"Failing Types: {string.Join("; ", result.FailingTypeNames ?? Array.Empty<string>())}"); (reference).

[BenMorris]

I recognise that returning null is regarded as a bad habit, but I'm reluctant to introduce a breaking change to the public interface at this point.

I'll keep this pull request open so it can be swept up when there's another major release - that will make it easier to signpost this kind of change so we don't break anybody's tests unnecessarily.

[lkurzyniec]

Fair enough, thx.

[jgveire]

I would really like this change because the first time I used this package I got a null reference exception because I expected failing types to be an empty array. I agree with @lkurzyniec that this is a common practice.