Handle option void type

- There's been no explicit support having argument void type
  as you would not be able to use with annotation model but
  surely can be wrapped as Type with CommandRegistration.
- For now change CommandInfoModel so that it uses empty string
  which is i.e. expected in help options which are just
  used as flags.
- It's outside of this commit to change parser to fail if
  user gives an argument value for this type of options.
- Fixes #586

spring-shell-core/src/main/java/org/springframework/shell/command/CommandRegistration.java

@@ -1141,7 +1141,8 @@ public interface CommandRegistration {
 			if (helpOptionsSpec != null) {
 				String[] longNames = helpOptionsSpec.longNames != null ? helpOptionsSpec.longNames : null;
 				Character[] shortNames = helpOptionsSpec.shortNames != null ? helpOptionsSpec.shortNames : null;
-				options.add(CommandOption.of(longNames, shortNames, "help for " + command));
+				options.add(CommandOption.of(longNames, shortNames, "help for " + command,
+						ResolvableType.forType(void.class)));
 			}
 			return options;
 		}