Running empty refrigerator
A clean refrigerator will not smell. The air inside would be entrapped and accumulate no other odors. If not fully clean, anything inside that might decompose would not do so at a suitably low temperature. Clean is still better than cool and old, however.

There is no harm in powering off the refrigerator. One can place a magnet on the door frame to prevent complete closing, which would allow any minor odors to escape. I have a dorm sized refrigerator configured in this manner and it does not smell. The door is about 12 mm open (1/2") and when started after three years of being unplugged, cooled properly and ran normally.

You will save energy at the warmest setting but not what I would call considerable. With the door closed and not opened for extended periods, the compressor should not operate as frequently as when in use, but will operate to bring the temperature back to the dial setting as needed. Temperatures outside the refrigerator and the insulation quality comes into play for that determination.

Not a duplicate, but another post to contribute to your question:
Unplugging vs running an empty refrigerator