Fix for DisposeAsync when maybe null

[JohanLarsson]

Before:

#nullable enable
namespace N
{
    using System;
    using System.IO;
    using System.Threading.Tasks;

    public class C : IAsyncDisposable
    {
        ↓private readonly IAsyncDisposable? disposable = File.OpenRead(string.Empty);

        public ValueTask DisposeAsync()
        {
            return default(ValueTask);
        }
    }
}

Not sure how we should generate the null check.
Only offering fix when notnull for now. Nullable case is likely rare.

[jeremyVignelles]

Nullable case is likely rare.

I have a lot of them :)

I spawn a process in a StartAsync() method, but my process is null before reaching StartAsync(), which made me do this:

if(this._process !=null)
{
   await this._process.DisposeAsync();
}

It might not be thread-safe, but for my use case it's ok.

A safer way might be to use Interlocked.Exchange()