Add default implementation to OpenAPI's new Description property to prevent a breaking change (#60937)

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

* Apply suggestions from code review

Co-authored-by: Safia Abdalla <safia@safia.rocks>

* Set default implementation on Description in IApiResponseMetadataProvider to prevent casting errors

---------

Co-authored-by: Safia Abdalla <safia@safia.rocks>

Author: sander1095

Diff for: 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 => null;
 
     /// <summary>
     /// Gets the content types supported by the metadata.

Diff for: src/Mvc/Mvc.Core/src/ApiExplorer/IApiResponseMetadataProvider.cs

@@ -20,7 +20,7 @@ public interface IApiResponseMetadataProvider : IFilterMetadata
     /// <summary>
     /// Gets the description of the response.
     /// </summary>
-    string? Description { get; }
+    string? Description => null;
 
     /// <summary>
     /// Gets the HTTP status code of the response.