Lift restriction on empty JpaEntityGraph attributes.

This commit removes a check preventing EntityGraphs from being created for JpaEntityGraphs without attributes.

Original pull request: #3684
Closes #3682

Author: christophstrobl

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/query/Jpa21Utils.java

@@ -129,7 +129,6 @@ private static EntityGraph<?> createDynamicEntityGraph(EntityManager em, JpaEnti
 		Assert.notNull(em, "EntityManager must not be null");
 		Assert.notNull(jpaEntityGraph, "JpaEntityGraph must not be null");
 		Assert.notNull(entityType, "Entity type must not be null");
-		Assert.isTrue(jpaEntityGraph.isAdHocEntityGraph(), "The given " + jpaEntityGraph + " is not dynamic");
 
 		EntityGraph<?> entityGraph = em.createEntityGraph(entityType);
 		configureFetchGraphFrom(jpaEntityGraph, entityGraph);

spring-data-jpa/src/test/java/org/springframework/data/jpa/repository/query/Jpa21UtilsTests.java

@@ -50,6 +50,7 @@
  * @author Mark Paluch
  * @author Jens Schauder
  * @author Krzysztof Krason
+ * @author Christoph Strobl
  */
 @ExtendWith(SpringExtension.class)
 @ContextConfiguration("classpath:application-context.xml")
@@ -166,6 +167,15 @@ void errorsOnUnknownProperties() {
 				em.createEntityGraph(User.class)));
 	}
 
+	@Test // GH-3682
+	void allowsEmptyGraph() {
+
+		EntityGraph<User> graph = em.createEntityGraph(User.class);
+		Jpa21Utils.configureFetchGraphFrom(new JpaEntityGraph("User.NoNamedEntityGraphAvailable", EntityGraphType.FETCH, new String[0]), graph);
+
+		Assertions.assertThat(graph.getAttributeNodes()).isEmpty();
+	}
+
 	/**
 	 * Lookup the {@link AttributeNode} with given {@literal nodeName} in the root of the given {@literal graph}.
 	 */