Fixed nullability warning due to missing annotation

Author: Sergio0694

Microsoft.Toolkit.HighPerformance/Extensions/IBufferWriterExtensions.cs

@@ -33,7 +33,7 @@ public static Stream AsStream(this IBufferWriter<byte> writer)
             {
                 // If the input writer is of type ArrayPoolBufferWriter<byte>, we can use the type
                 // specific buffer writer owner to let the JIT elide callvirts when accessing it.
-                var internalWriter = Unsafe.As<ArrayPoolBufferWriter<byte>>(writer);
+                var internalWriter = Unsafe.As<ArrayPoolBufferWriter<byte>>(writer)!;
 
                 return new IBufferWriterStream<ArrayBufferWriterOwner>(new ArrayBufferWriterOwner(internalWriter));
             }