Return null values from Connection visitor when expected

Prior to this commit, a schema declaring a `BookConnection` nullable
type would decorate the relevant data fetcher and return an
`EMPTY_CONNECTION` value (a full Connection instance with empty nodes
and edges) if the original DataFetcher returns `null`.

This is unexpected for applications because the type is declared as
nullable in the schema and the application returns a `null` value.

This commit ensures that `EMPTY_CONNECTION` is only returned as a value
if the Connection type is marked as non nullable, `BookConnection!`.

This change should only affect applications with custom pagination
support, as Spring Data never returns `null` for empty pages.

Closes gh-1295

Author: bclozel

spring-graphql/src/main/java/org/springframework/graphql/data/pagination/ConnectionFieldTypeVisitor.java

@@ -24,6 +24,7 @@
 
 import graphql.TrivialDataFetcher;
 import graphql.execution.DataFetcherResult;
+import graphql.language.NonNullType;
 import graphql.relay.Connection;
 import graphql.relay.DefaultConnection;
 import graphql.relay.DefaultConnectionCursor;
@@ -101,7 +102,7 @@ public TraversalControl visitGraphQLFieldDefinition(
 				}
 			}
 			else {
-				dataFetcher = new ConnectionDataFetcher(dataFetcher, this.adapter);
+				dataFetcher = new ConnectionDataFetcher(dataFetcher, this.adapter, fieldDefinition);
 				codeRegistry.dataFetcher(fieldCoordinates, dataFetcher);
 			}
 		}
@@ -190,10 +191,11 @@ public static ConnectionFieldTypeVisitor create(List<ConnectionAdapter> adapters
 
 	/**
 	 * {@code DataFetcher} decorator that adapts return values with an adapter.
-	 * @param delegate the datafetcher delegate
+	 * @param delegate the DataFetcher delegate
 	 * @param adapter the connection adapter to use
+	 * @param connectionFieldDefinition the field definition for the connection type
 	 */
-	record ConnectionDataFetcher(DataFetcher<?> delegate, ConnectionAdapter adapter) implements DataFetcher<Object> {
+	record ConnectionDataFetcher(DataFetcher<?> delegate, ConnectionAdapter adapter, GraphQLFieldDefinition connectionFieldDefinition) implements DataFetcher<Object> {
 
 		private static final Connection<?> EMPTY_CONNECTION =
 				new DefaultConnection<>(Collections.emptyList(), new DefaultPageInfo(null, null, false, false));
@@ -202,6 +204,7 @@ record ConnectionDataFetcher(DataFetcher<?> delegate, ConnectionAdapter adapter)
 		ConnectionDataFetcher {
 			Assert.notNull(delegate, "DataFetcher delegate is required");
 			Assert.notNull(adapter, "ConnectionAdapter is required");
+			Assert.notNull(connectionFieldDefinition, "GraphQLFieldDefinition is required");
 		}
 
 
@@ -232,9 +235,9 @@ private Object adaptDataFetcherResult(@Nullable Object value) {
 			}
 		}
 
-		private <T> Object adaptDataContainer(@Nullable Object container) {
+		private <T> @Nullable Object adaptDataContainer(@Nullable Object container) {
 			if (container == null) {
-				return EMPTY_CONNECTION;
+				return isConnectionTypeNullable() ? null : EMPTY_CONNECTION;
 			}
 
 			if (container instanceof Connection<?>) {
@@ -268,6 +271,13 @@ private <T> Object adaptDataContainer(@Nullable Object container) {
 			return new DefaultConnection<>(edges, pageInfo);
 		}
 
+		private boolean isConnectionTypeNullable() {
+			if (this.connectionFieldDefinition.getDefinition() != null) {
+				return !(this.connectionFieldDefinition.getDefinition().getType() instanceof NonNullType);
+			}
+			return true;
+		}
+
 	}
 
 }

spring-graphql/src/test/java/org/springframework/graphql/data/pagination/ConnectionFieldTypeVisitorTests.java

@@ -137,15 +137,35 @@ void customConnectionTypeIsPassedThrough() {
 		);
 	}
 
-	@Test // gh-707
-	void nullValueTreatedAsEmptyConnection() {
+	@Test // gh-707, gh-1295
+	void nullValueTreatedAsNullConnectionWhenNullable() {
 
 		Mono<ExecutionGraphQlResponse> response = GraphQlSetup.schemaResource(BookSource.paginationSchema)
 				.dataFetcher("Query", "books", environment -> null)
 				.connectionSupport(new ListConnectionAdapter())
 				.toGraphQlService()
 				.execute(BookSource.booksConnectionQuery(null));
 
+		ResponseHelper.forResponse(response).assertData("{\"books\":null}");
+	}
+
+	@Test // gh-1295
+	void nullValueTreatedAsEmptyConnectionWhenNonNullable() {
+		Mono<ExecutionGraphQlResponse> response = GraphQlSetup.schemaContent("""
+						type Query {
+							books(first:Int, after:String): BookConnection!
+						}
+
+						type Book {
+							id: ID
+							name: String
+						}
+						""")
+				.dataFetcher("Query", "books", environment -> null)
+				.connectionSupport(new ListConnectionAdapter())
+				.toGraphQlService()
+				.execute(BookSource.booksConnectionQuery(null));
+
 		ResponseHelper.forResponse(response).assertData(
 				"{\"books\":{" +
 						"\"edges\":[]," +