Kidney yin deficiency

"Empty heat" is a term used to describe the concept of "kidney yin deficiency" - so they are both saying the same thing somewhat.  Empty heat is the idea of signs of heat, such as nightsweats, but coming from a deficiency.  In this case a deficiency of yin - the cooling aspect of the body.  This is opposed to full-heat or a yang or fire condition that is coming from excess.  How you approach treatment depends greatly on the clarify of the diagnosis.

For a general description of empty heat, I would suggest you read "My Kidneys are What?" - which is a general introduction to the concepts of "kidney" system imbalances from a Chinese medicine perspective.