Fix ShellOption with required

- Where applicable mark option required so that
  user is notified instead of blindly using null.
- Fixes #436

spring-shell-standard/src/main/java/org/springframework/shell/standard/StandardMethodTargetRegistrar.java

@@ -140,6 +140,9 @@ public class StandardMethodTargetRegistrar implements MethodTargetRegistrar, App
 									&& !ObjectUtils.nullSafeEquals(so.defaultValue(), ShellOption.NULL)) {
 								optionSpec.defaultValue(so.defaultValue());
 							}
+							else {
+								optionSpec.required();
+							}
 						}
 					}
 					else {