Fix S2259 FP: Should not report in switch expression after a null check

[pavel-mikula-sonarsource]

First null check should build NotNull constraint in "else" branch. Other checks should take that into account

public string NullCheck(object arg) =>
    arg switch
    {
        null => throw new ArgumentNullException(nameof(arg)),
        { } when IsInstance(arg) => "42",  // This somehow learns possible Null when it's already known to be NotNull
        _ => throw new InvalidOperationException("Unexpected type: " + arg.GetType().Name)  // Noncompliant FP
    };

private static bool IsInstance(object arg) => false;