Unexpected nullable warning in object initializer member using extension Add method

**Version Used**:

**Steps to Reproduce**: [.NET Lab](https://lab.razor.fyi/#fVHRihMxFGVhVzRPsl9wwIdpX2bFBxFrC6UUWVEQt0-KaDpzZwxk7pTkZrelzB_4Lz75Bz4K_onfIOlk266IeUhI7j3n5J6jfpwp9da1tdNNXvjz72ePOFirl5ZAHA8VvOEaVxsv1Izu3PJZay0VYlr2-Uticqb4T8dl0wSJlCOlCqu9RxJWWwUAXrSYAtetKfFGGx4Md899Ma7LOYeGXKR44cUZricwQo3HGB8-jvZ9nzAG0w1mieEuy44pobYJ3-HiAoFpvaJCqMSNdhynmF09e_rk8XNMXR0aYkFbQTYrQvaPv2QoNHMrWBKCpxJV67DSTjck5JBZ4yU7MLw2Xg5Qw8jma8lpLcTetHxbGhz3DTOUgSAtSlNV5IgL8hErXwh9cMYa2UQVR6kBUc_newO63qlOdbc5zFICq7C0psCxYrJqi5pkBB-3LtndKZUi61nma0k8hyEW6f-LCeL4fyeaBHeRT8tycOzqYoJrbQP5IcY9Op-W5TvNNQ1SYT_I-WmjDb-6R861zr9_cP_3t19ff1YPTz-frE_-AA)

```cs
#nullable enable
using System;
using System.Collections.Generic;
using System.Collections.Immutable;

class Program
{
static void Main()
{
IEnumerable items = [];
_ = new C()
{
Items = { items } // unexpected warning CS8620: Argument of type 'IEnumerable' cannot be used for parameter 'list' of type 'List' in 'Ext.extension(List)' due to differences in the nullability of reference types.
};
}
}

class C
{
public List Items { get; set; } = [];
}

static class Ext
{
extension(List list)
{
public void Add(IEnumerable values) => list.AddRange(values);
}
}
```

**Expected Behavior**: No warnings are reported

**Actual Behavior**: A nullable warning is reported for 'items'. Note that the parameter which is being referenced is also unexpected,

The bug is may be related to the adjustments which are made to extension calls in order to include the receiver argument. We would want to ensure that the scenario behaves similarly to an classic `public static void Add(this List list, IEnumerable values) => ...;` extension method.