Fix null message in internal static string? ExtendedMessage(string methodName, string? message, string actualExpression, string constraintExpression)

[OsirisTerje]

Ref comment in PR #4430

The message comes from the _getExceptionMessageoverloads which are declared asFunc<string?>. Also there are some NUnit tests (e.g. LifeCycleAttributeTest) that pass in result.Messagewhich is also declaredstring?`.
If we want to change all that, I would do that in a separate PR.

[manfred-brands]

@OsirisTerje I started in this.

[OsirisTerje]

@manfred-brands Awesome!