Assert.That no longer allows 'null' message

[manfred-brands]

I encountered NUnit extension methods that have a signature with: string? message = null which eventually calls into some version of Assert.That(actual, constaint, message). These no longer compile.

[manfred-brands]

Can be fixed by allowing null in the implicit operator:

public static implicit operator NUnitString(string? message) => new(message);

NUnitString already supports null as that will be the value when not passed at all.