Fix Added MemberNotNullWhen in IApiResponse (#1483)

* Added MemberNotNullWhen in IApiResponse

* Update ApiResponse.cs

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Co-authored-by: Glenn <[email protected]>

Author: AlbertoMonteiro

Refit/ApiResponse.cs

@@ -134,6 +134,10 @@ public interface IApiResponse : IDisposable
         /// <summary>
         /// Indicates whether the request was successful.
         /// </summary>
+#if NET5_0_OR_GREATER
+        [MemberNotNullWhen(true, nameof(ContentHeaders))]
+        [MemberNotNullWhen(false, nameof(Error))]
+#endif
         bool IsSuccessStatusCode { get; }
 
         /// <summary>