Fix NPE for non-required missing input arguments

Prior to this commit, a missing, non-required input argument would throw
an NullPointerException instead of returning `null` or
`Optional.empty()`.

Fixes gh-144

spring-graphql/src/main/java/org/springframework/graphql/data/method/annotation/support/ArgumentMethodArgumentResolver.java

@@ -74,7 +74,7 @@ public class ArgumentMethodArgumentResolver implements HandlerMethodArgumentReso
 			if (annotation.required()) {
 				throw new MissingArgumentException(name, parameter);
 			}
-			returnValue(rawValue, parameterType.getType());
+			return returnValue(rawValue, parameterType.getType());
 		}
 
 		if (CollectionFactory.isApproximableCollectionType(rawValue.getClass())) {

spring-graphql/src/test/java/org/springframework/graphql/data/method/annotation/support/ArgumentMethodArgumentResolverTests.java

@@ -96,6 +96,16 @@ class ArgumentMethodArgumentResolverTests {
 		assertThat(result).isNotNull().isInstanceOf(Long.class).isEqualTo(42L);
 	}
 
+	@Test
+	void shouldNotFailIfArgumentNotRequired() throws Exception {
+		Method findByKeywords = ClassUtils.getMethod(BookController.class, "findByKeywords", List.class);
+		String payload = "{ }";
+		DataFetchingEnvironment environment = initEnvironment(payload);
+		MethodParameter methodParameter = getMethodParameter(findByKeywords, 0);
+		Object result = resolver.resolveArgument(methodParameter, environment);
+		assertThat(result).isNull();
+	}
+
 	@Test
 	void shouldResolveListOfJavaBeansArgument() throws Exception {
 		Method addBooks = ClassUtils.getMethod(BookController.class, "addBooks", List.class);
@@ -137,6 +147,11 @@ class ArgumentMethodArgumentResolverTests {
 			return null;
 		}
 
+		@QueryMapping
+		public Book findByKeywords(@Argument(required = false) List<Keyword> keywords) {
+			return null;
+		}
+
 		@MutationMapping
 		public Book addBook(@Argument BookInput bookInput) {
 			return null;
@@ -172,4 +187,17 @@ class ArgumentMethodArgumentResolverTests {
 		}
 	}
 
+	static class Keyword {
+
+		String term;
+
+		public String getTerm() {
+			return this.term;
+		}
+
+		public void setTerm(String term) {
+			this.term = term;
+		}
+	}
+
 }