How to mock (OptionSetMetadata)retrieveOptionSetResponse.OptionSetMetadata;?

// Access the retrieved OptionSetMetadata.
OptionSetMetadata retrievedOptionSetMetadata =
(OptionSetMetadata)retrieveOptionSetResponse.OptionSetMetadata;

OptionSetMetadata is null . how to mock the above code with out changing the plugin code. 

namespace Microsoft.Xrm.Sdk.Messages
{
//
// Summary:
// Contains the response from the Microsoft.Xrm.Sdk.Messages.RetrieveOptionSetRequest
// class.
[System.Runtime.Serialization.DataContractAttribute(Namespace = "http://schemas.microsoft.com/xrm/2011/Contracts")]
public sealed class RetrieveOptionSetResponse : OrganizationResponse
{
//
// Summary:
// Initializes a new instance of the Microsoft.Xrm.Sdk.Messages.RetrieveOptionSetResponse
// class.
public RetrieveOptionSetResponse();

//
// Summary:
// Gets the metadata for the global option set.
//
// Returns:
// Type: Microsoft.Xrm.Sdk.Metadata.OptionSetMetadataBase The metadata for the global
// option set.
public OptionSetMetadataBase OptionSetMetadata { get; }
}
}

OptionSetMetadataBase OptionSetMetadata { get; } this is having only set property and OptionSetMetadataBase  is abstract class.