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 {

spring-shell-standard/src/test/java/org/springframework/shell/standard/StandardMethodTargetRegistrarTests.java

@@ -64,7 +64,7 @@ public class StandardMethodTargetRegistrarTests {
 
 	@Test
 	public void testRegistrations() {
-		applicationContext = new AnnotationConfigApplicationContext(Sample.class);
+		applicationContext = new AnnotationConfigApplicationContext(Sample1.class);
 		registrar.setApplicationContext(applicationContext);
 		registrar.register(catalog);
 		Map<String, CommandRegistration> registrations = catalog.getRegistrations();
@@ -74,6 +74,7 @@ public class StandardMethodTargetRegistrarTests {
 		assertThat(registrations.get("say-hello").getAvailability()).isNotNull();
 		assertThat(registrations.get("say-hello").getOptions()).hasSize(1);
 		assertThat(registrations.get("say-hello").getOptions().get(0).getLongNames()).containsExactly("what");
+		assertThat(registrations.get("say-hello").getOptions().get(0).isRequired()).isTrue();
 
 		assertThat(registrations.get("hi")).isNotNull();
 		assertThat(registrations.get("hi").getAvailability()).isNotNull();
@@ -83,7 +84,7 @@ public class StandardMethodTargetRegistrarTests {
 	}
 
 	@ShellComponent
-	public static class Sample {
+	public static class Sample1 {
 
 		@ShellMethod("some command")
 		public String sayHello(String what) {
@@ -96,6 +97,28 @@ public class StandardMethodTargetRegistrarTests {
 		}
 	}
 
+	@Test
+	public void testOptionRequiredWithAnnotation() {
+		applicationContext = new AnnotationConfigApplicationContext(Sample2.class);
+		registrar.setApplicationContext(applicationContext);
+		registrar.register(catalog);
+		Map<String, CommandRegistration> registrations = catalog.getRegistrations();
+		assertThat(registrations).hasSize(1);
+
+		assertThat(registrations.get("say-hello")).isNotNull();
+		assertThat(registrations.get("say-hello").getOptions()).hasSize(1);
+		assertThat(registrations.get("say-hello").getOptions().get(0).isRequired()).isTrue();
+	}
+
+	@ShellComponent
+	public static class Sample2 {
+
+		@ShellMethod("some command")
+		public String sayHello(@ShellOption String what) {
+			return "hello " + what;
+		}
+	}
+
 	@Test
 	public void testAvailabilityIndicators() {
 		applicationContext = new AnnotationConfigApplicationContext(SampleWithAvailability.class);