Minio client 6.0.5 crashes with NullReferenceException

[asmirnov82]

Minio client crashes with NullReference exception in case of ResponseResult has empty Exception field (for example in case of Service unavailable response).

Version: 6.0.5

System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=Minio
StackTrace:
at Minio.MinioClient.ParseErrorNoContent(ResponseResult response)
at Minio.MinioClient.ParseError(ResponseResult response)
at Minio.Handlers.DefaultErrorHandler.Handle(ResponseResult response)
at Minio.RequestExtensions.d__2.MoveNext() in C:\Sources\OpenSource\minio-dotnet\Minio\RequestExtensions.cs:line 69

This exception was originally thrown at this call stack:
Minio.MinioClient.ParseErrorNoContent(Minio.DataModel.Result.ResponseResult) in MinioClient.cs
Minio.MinioClient.ParseError(Minio.DataModel.Result.ResponseResult) in MinioClient.cs
Minio.Handlers.DefaultErrorHandler.Handle(Minio.DataModel.Result.ResponseResult) in DefaultErrorHandler.cs
Minio.RequestExtensions.ExecuteTaskAsync(Minio.IMinioClient, Minio.HttpRequestMessageBuilder, System.Type, bool, System.Threading.CancellationToken) in RequestExtensions.cs

Possibly related to #1316

Similar issue is fixed in PR #1330