The element mainClass is empty because the property storm.topology has no value, this is why you get an error.
You have to pass the storm.topology parameter instead of mainClass :
mvn -e -f m2-pom.xml compile exec:java -Dstorm.topology=storm.starter.MainTopology
See the Maven section of the readme of this example for more information: https://github.com/nathanmarz/storm-starter