Add DiagnosticSuppressor for Nullable<T>

[manfred-brands]

Currently the following is not suppressed:

int? value = GetValue();
Assert.That(value.HasValue, Is.True);
Assert.That(value.Value, Is.EqualTo(42));

static int? GetValue() => 42;

We need to update to DiagnosticSuppressor to detected Nullable<T> and .HasValue

[manfred-brands]

Workaround is:

int? value = GetValue();
Assert.That(value, Is.Not.Null);
Assert.That(value.Value, Is.EqualTo(42));

static int? GetValue() => 42;