How can I tell if an XML block is 'empty'

Is there a quick way to do this? If I receive an 'empty' XML fragment eg <Candidates></Candidates> - return, or else process the data inside.
Logic vastly reduced - so what I'm asking for is a quick way to see if XML = '<Candidates></Candidates>' in a SQL stored PROC.

Thanks,
Doug

CREATE PROCEDURE [dbo].[Receive_XML]
(
  @CandidateXML XML
)
AS
BEGIN	
	SET NOCOUNT ON;

	IF NULLIF( CAST( @CandidateXML AS NVARCHAR), N'<Candidates></Candidates>' ) IS NOT NULL
	BEGIN
		-- Do Processing
	END
        ELSE
        BEGIN
                -- Return - nothing to process
        END
END

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