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-samples/src/main/java/org/springframework/shell/samples/e2e/OptionTypeCommands.java

@@ -230,4 +230,26 @@ public class OptionTypeCommands extends BaseE2ECommands {
 				.and()
 			.build();
 	}
+
+	//
+	// Void
+	//
+
+	@Bean
+	public CommandRegistration optionTypeVoidRegistration(Supplier<CommandRegistration.Builder> builder) {
+		return builder.get()
+			.command(REG, "option-type-void")
+			.group(GROUP)
+			.withOption()
+				.longNames("arg1")
+				.type(void.class)
+				.and()
+			.withTarget()
+				.function(ctx -> {
+					return "Hello ";
+				})
+				.and()
+			.build();
+	}
+
 }