No, it is not necessary to put water in an empty can before putting it in the microwave oven. In fact, you should never put any metal objects, including cans, in a microwave oven. Microwaving metal can cause sparks and potentially start a fire or damage the microwave.
The microwave works by emitting electromagnetic waves that excite water molecules in the food, causing them to heat up. Metal objects, like aluminum cans, can reflect these waves and create electrical currents, leading to dangerous consequences.
If you want to heat the contents of a can, it's best to empty the contents into a microwave-safe container before heating. Always follow the guidelines provided by the manufacturer and exercise caution to ensure safe microwave use.