`out var` treated as non-nullable

`out var` is treated as non-nullable, regardless of parameter declaration.
```c#
class C
{
    static void TryGet(out string? s)
    {
        s = null;
    }
    static void Main()
    {
        TryGet(out var s); // string with null assignment warning
        int n = s.Length;  // no warning
    }
}
```