Apply suggestion from @dgriffon

Co-authored-by: David Griffon <dgriffon@jahia.com>

Author: anttip

src/main/java/graphql/annotations/dataFetchers/MethodDataFetcher.java

@@ -165,8 +165,8 @@ private Object buildArg(Type p, GraphQLType graphQLType, Object arg) {
             Type subType = ((ParameterizedType) p).getActualTypeArguments()[0];
             Object val = buildArg(subType, graphQLType, arg);
             // add Optional wrapper if needed
-            if (val != null && ((ParameterizedType) p).getRawType() == Optional.class) {
-                return Optional.of(val);
+            if (((ParameterizedType) p).getRawType() == Optional.class) {
+                return Optional.ofNullable(val);
             }
             return val;
         } else {