title: SQL Server: WHERE clause to ignore blank GUID
body: <pre><code>SELECT ...
  FROM ...
  WHERE CASE WHEN @ActivityUID = '00000000-0000-0000-0000-000000000000' 
      THEN
          -- do nothing
      ELSE
          (Activity.ActivityUID = @ActivityUID)
      END
  </code></pre>
  
  <p>When a blank Guid is passed in via ActivityUID then I want the WHERE statement to ignore <code>Activity.ActivityUID = @ActivityUID</code></p>