what do empty 8th house and 12th house in a single chart signify????????

You need to look at alot of other things too like the cusps on the houses. The chart as a whole. Usually the position of the ruler of the sign on the cusp of the house in question will point to more interpretation. I think when you have no planets inside a house it would mean that you have no problems dealing with / accepting the issues of that house. The more planets inside a house, the more hang ups, the more difficulty you experience with that house. Or the more important the issues of that house become to you.

It is a very rare chart that doesn't have a house or two or three or four without planets.  I agree with the above post.  Also consider WHERE these houses are located....above or below the horizon.  Also, where are the other planets, and how do they make aspects to the cusps of these "empty" houses.  These houses will become significant as transiting planets enter them and make ASPECTS to the natal planets in other houses.
Also, are you so very sure that they are empty?  Have you calculated the positions of all nodes and minor "planets" such as Chiron?
To make this simple: we'd need more information (the entire natal chart) to even begin to answer "what this means?"

Dear friends,

If I find no planet in the house of birth chart, you should consider the zodiac sign on cusp and in which house its ruler can be found and consider also what is the matter represented by zodiac sign. For example: 8th house ruled by Sagitarius. Sagitarius represents travel. So it means that death can be occured during travelling. If 8th house ruled by Cancer, it means that death can be occured at home;because the ruler of Cancer is Moon, which represents home. May this information helpfull to you.

Regards,

Ivan T.