It is a minor issue in the EC2 Plugin implementation. 
instanceCapStr in https://github.com/jenkinsci/ec2-plugin/blob/master/src/main/java/hudson/plugins/ec2/EC2Cloud.java is designed to be optional, and it supports empty strings in the constructor. But it still fails with NPE when a field is not passed from the JCasC configuration.
 
I consider it is as an important issue, because the field is not exported by default.

The issue can be fixed by a small patch to properly handle nulls in the constructor arguments