Should not report warning for second dereference of nullable value

The second warning is redundant and incorrect:
```
class Program
{
    static void F(string? s)
    {
        int n = s.Length; // warning: 's' may be null
        if (s.Length > 0) // warning: 's' may be null
        {
        }
    }
}
```
```
(5,17): warning CS8602: Possible dereference of a null reference.
(6,13): warning CS8602: Possible dereference of a null reference.
```