Fix S2259 FP/FN: Do not raise on Nullable members

[pavel-mikula-sonarsource]

In the context of S2259 and S3655, we need to split their responsibility for handling nullable types.

S2259 should not raise on

• nullable.HasValue
• nullable.Value
• nullable.GetValueOrDefault()
• nullable.Equals(..)
• nullable.ToString()
• nullable.GetHashCode()

as those are safe to call when nullable has ObjectConstraint.NullValue, or handled by S3655.

It should still raise on

• nullable.GetType()