A lack of wet spots or soiled bedding in your bearded dragon's enclosure does not indicate a health problem. He simply has a specialized method of releasing toxins and waste from his body.
A bearded dragon does not urinate in the same way as humans or most other animals. Instead of releasing toxins and waste in a liquid form, he produces a dry, white powdery substance. This substance is uric acid, and is essentially a concentrated form of urine without the liquid portion. It is often seen in conjunction with fecal waste. This method of removing toxins from the body without also releasing moisture helps a bearded dragon survive in his natural desert environment, since he is not losing moisture or risking dehydration each time that he urinates.