Handling for empty representations list

Closes gh-1057

Author: rstoyanchev

spring-graphql/src/main/java/org/springframework/graphql/data/federation/EntitiesDataFetcher.java

@@ -70,6 +70,9 @@ public Mono<DataFetcherResult<List<Object>>> get(DataFetchingEnvironment env) {
 			return Mono.error(new RepresentationException(
 					Collections.emptyMap(), "Missing \"representations\" argument"));
 		}
+		if (representations.isEmpty()) {
+			return Mono.just(DataFetcherResult.<List<Object>>newResult().data(Collections.emptyList()).build());
+		}
 
 		Set<String> batchedTypes = new HashSet<>();
 		List<Mono<?>> monoList = new ArrayList<>();

spring-graphql/src/test/java/org/springframework/graphql/data/federation/EntityMappingInvocationTests.java

@@ -115,6 +115,14 @@ void fetchEntitiesWithExceptions() {
 		assertAuthor(6, "George", "Orwell", helper);
 	}
 
+	@Test // gh-1057
+	void fetchEntitiesWithEmptyList() {
+		Map<String, Object> vars = Map.of("representations", Collections.emptyList());
+		ResponseHelper helper = executeWith(BookController.class, vars);
+
+		assertThat(helper.toEntity("_entities.length()", Integer.class)).isEqualTo(0);
+	}
+
 	@ValueSource(classes = {BookListController.class, BookFluxController.class})
 	@ParameterizedTest
 	void batching(Class<?> controllerClass) {