Fix NullPointerException in SchemaMapping inspector

Prior to this commit, the new nullness checks in the schema mapping
inspection (see gh-1220) would fail for null field definitions. While
not common in schemas, such types can be cnotributed by the Federation
support for the JVM.

This commit ensures that such types are considered as nullness
"unspecified" and does not break the inspection process.

Fixes gh-1333

Author: bclozel

spring-graphql/src/main/java/org/springframework/graphql/execution/SchemaMappingInspector.java

@@ -34,6 +34,7 @@
 import java.util.function.Function;
 import java.util.function.Predicate;
 
+import graphql.language.FieldDefinition;
 import graphql.language.NonNullType;
 import graphql.language.Type;
 import graphql.schema.DataFetcher;
@@ -383,7 +384,11 @@ private void checkDataFetcherRegistrations() {
 	}
 
 	private Nullness resolveNullness(GraphQLFieldDefinition fieldDefinition) {
-		return resolveNullness(fieldDefinition.getDefinition().getType());
+		FieldDefinition definition = fieldDefinition.getDefinition();
+		if (definition != null) {
+			return resolveNullness(definition.getType());
+		}
+		return Nullness.UNSPECIFIED;
 	}
 
 	private Nullness resolveNullness(Type type) {

spring-graphql/src/test/java/org/springframework/graphql/execution/SchemaMappingInspectorNullnessTests.java

@@ -18,6 +18,17 @@
 
 import java.util.concurrent.CompletableFuture;
 
+import graphql.Scalars;
+import graphql.schema.GraphQLFieldDefinition;
+import graphql.schema.GraphQLNonNull;
+import graphql.schema.GraphQLObjectType;
+import graphql.schema.GraphQLSchema;
+import graphql.schema.GraphQLSchemaElement;
+import graphql.schema.GraphQLTypeVisitorStub;
+import graphql.schema.SchemaTransformer;
+import graphql.schema.idl.SchemaGenerator;
+import graphql.util.TraversalControl;
+import graphql.util.TraverserContext;
 import org.jspecify.annotations.NonNull;
 import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.NullUnmarked;
@@ -349,6 +360,24 @@ void reportIsEmptyWhenSchemaFieldNonNullAndAsyncTypeNullable() {
 			assertThatReport(report).isEmpty();
 		}
 
+		@Test
+		void doesNotFailWhenNullFieldDefinitionType() {
+			String schemaContent = """
+						type Query {
+							bookById(id: ID): DefaultBook
+						}
+						type DefaultBook {
+							id: ID
+							title: String
+						}
+					""";
+
+			GraphQLSchema schema = SchemaGenerator.createdMockedSchema(schemaContent);
+			schema = SchemaTransformer.transformSchema(schema, new NullFieldTypeVisitor());
+			SchemaReport report = inspectSchema(schema, initializer -> { }, DefaultBookController.class);
+			assertThatReport(report).containsUnmappedFields("DefaultBook", "_service");
+		}
+
 		@Controller
 		@NullUnmarked
 		static class DefaultBookController {
@@ -382,9 +411,6 @@ static class NonNullBookController {
 
 		}
 
-
-
-
 		@Controller
 		@NullUnmarked
 		static class NullableAsyncBookController {
@@ -400,6 +426,26 @@ record Book(String id, String title) {
 
 		}
 
+		static class NullFieldTypeVisitor extends GraphQLTypeVisitorStub {
+			@Override
+			public TraversalControl visitGraphQLFieldDefinition(GraphQLFieldDefinition node, TraverserContext<GraphQLSchemaElement> context) {
+				if (node.getName().equals("title")) {
+					GraphQLObjectType type = GraphQLObjectType.newObject()
+							.name("_Service")
+							.field(
+									GraphQLFieldDefinition.newFieldDefinition()
+											.name("sdl")
+											.type(new GraphQLNonNull(Scalars.GraphQLString))
+											.build())
+							.build();
+					GraphQLFieldDefinition field = GraphQLFieldDefinition.newFieldDefinition()
+							.name("_service").type(GraphQLNonNull.nonNull(type)).build();
+					return insertAfter(context, field);
+				}
+				return TraversalControl.CONTINUE;
+			}
+		}
+
 	}
 
 	@Nested

spring-graphql/src/test/java/org/springframework/graphql/execution/SchemaMappingInspectorTestSupport.java

@@ -43,8 +43,12 @@ protected SchemaReport inspectSchema(String schemaContent, Class<?>... controlle
 
 	protected SchemaReport inspectSchema(
 			String schemaContent, Consumer<SchemaMappingInspector.Initializer> consumer, Class<?>... controllers) {
-
 		GraphQLSchema schema = SchemaGenerator.createdMockedSchema(schemaContent);
+		return inspectSchema(schema, consumer, controllers);
+	}
+
+	protected  SchemaReport inspectSchema(
+			GraphQLSchema schema, Consumer<SchemaMappingInspector.Initializer> consumer, Class<?>... controllers) {
 		RuntimeWiring runtimeWiring = createRuntimeWiring(controllers);
 		SchemaMappingInspector.Initializer initializer = SchemaMappingInspector.initializer();
 		consumer.accept(initializer);