DelimitedOptionHandler.printDefaultValue() throws NPE (when there are no defaults)

[atnak]

This causes CmdLineParser.printUsage() to throw NPE, and appears to be a regression of "default values in usage".

It affects classes subclassing DelimitedOptionHandler such as MultiPathOptionHandler and MultiFileOptionHandler.

Minimal code to reproduce

public class Foo {

	@Argument(usage = "Foo", handler = MultiPathOptionHandler.class)
	List<Path> paths;

	public static void main(String[] args) {
		new CmdLineParser(new Foo()).printUsage(System.out);
	}
}