Fix ReSharper's "UseNullPropagation"

Ix.NET/Source/System.Linq.Async.Queryable/System/Linq/AsyncEnumerableRewriter.cs

@@ -291,10 +291,7 @@ namespace System.Linq
                         }
                     }
 
-                    if (list != null)
-                    {
-                        list.Add(expression);
-                    }
+                    list?.Add(expression);
                 }
 
                 //

Ix.NET/Source/System.Linq.Async/System/Linq/Disposables.cs

@@ -47,10 +47,7 @@ namespace System.Linq
                 d1.Dispose();
 
                 var d2 = Interlocked.Exchange(ref _d2, null);
-                if (d2 != null)
-                {
-                    d2.Dispose();
-                }
+                d2?.Dispose();
             }
         }
     }