Running the engine out of fuel is running it Lean!
You should never run any engine out of fuel. It can result in damage, as the mixture will go lean. On 4 strokes because they have so much torque they can spit the prop and spinner. In your case you are very lucky. Instead of the prop coming off and flying at you or someone else, the double jam nut protected you. It did not however protect the engines thrust washer.

The engine backfired and the prop slipped. This is common of the damage caused by a lean run. Hopefully there is not any more damage internally to the engine.

A 20 min run time is usually recommended before you try to fly the engine. 20mins of run time doesn't mean that it will run for 20mins on a single tank of fuel/ This would always depend on how large of tank you have.

A good practice is to run the engine for about 5-8 mins at time then let it cool completely before you start it again. I don't always do this personally but its a standard practice for breaking in an engine.