Fix S8969 FP: Implicit conversion is not taken into account

[drieseng]

Description

When a given type implements an implicit conversion that can return null, S8969 does not take this into consideration.

Reproducer

internal sealed class Foo
{
    public static implicit operator string?(Foo foo)
    {
        return null;
    }

    public static void Do(Foo foo)
    {
        string x = foo!; // S8969 is reported while the implicit operator can effectively return null
    }
}

Product and Version

SonarAnalyzer.CSharp version 10.33.0.1635