Add default implementation to Description to prevent a breaking change when consumers cast this type and get exceptions about description being null

Author: sander1095

src/Http/Http.Abstractions/src/Metadata/IProducesResponseTypeMetadata.cs

@@ -21,7 +21,7 @@ public interface IProducesResponseTypeMetadata
     /// <summary>
     /// Gets the description of the response.
     /// </summary>
-    string? Description { get; }
+    string? Description { get; } = null;
 
     /// <summary>
     /// Gets the content types supported by the metadata.