[HOT-304948] Fix empty namespace emitted as null

lib/src/main/java/graphql/nadel/result/NadelResultMerger.kt

@@ -62,7 +62,13 @@ internal object NadelResultMerger {
 
             // Ensure field is present in result
             if (topLevelResultKey.value !in data) {
-                data[topLevelResultKey.value] = null
+                val topLevelField = topLevelFields.first { it.resultKey == topLevelResultKey.value }
+                data[topLevelResultKey.value] =
+                    if (isNamespacedField(topLevelField, engineSchema) && children.isEmpty()) {
+                        mutableMapOf<String, Any?>()
+                    } else {
+                        null
+                    }
             }
 
             if (children.isNotEmpty()) {

test/src/test/kotlin/graphql/nadel/tests/next/fixtures/namespaced/EmptyNamespaceIsEmptyObjectTest.kt

@@ -0,0 +1,61 @@
+package graphql.nadel.tests.next.fixtures.namespaced
+
+import graphql.nadel.NadelExecutionHints
+import graphql.nadel.tests.next.NadelIntegrationTest
+
+/**
+ * Regression test for the bug where an empty namespace was materialised as `"namespace": null`.
+ *
+ * When every child of a namespaced field is skipped (e.g. `@skip(if: true)`), the namespace has no
+ * children and no service call is made. It must be materialised as an empty object `"namespace": {}`,
+ * NOT as `"namespace": null` (which tripped a downstream non-null handler, turning the whole
+ * response into `data: null`).
+ *
+ * Expected result: `{ "hello": "world", "namespace": {} }`.
+ */
+class EmptyNamespaceIsEmptyObjectTest : NadelIntegrationTest(
+    query = """
+        query {
+          hello
+          namespace {
+            foo @skip(if: true)
+          }
+        }
+    """.trimIndent(),
+    services = listOf(
+        Service(
+            name = "monolith",
+            overallSchema = """
+                type Query {
+                  hello: String
+                  namespace: NamespaceQuery @namespaced
+                }
+                type NamespaceQuery {
+                  foo: String
+                }
+            """.trimIndent(),
+            underlyingSchema = """
+                type Query {
+                  hello: String
+                  namespace: NamespaceQuery
+                }
+                type NamespaceQuery {
+                  foo: String
+                }
+            """.trimIndent(),
+            runtimeWiring = { wiring ->
+                wiring
+                    .type("Query") { type ->
+                        type
+                            .dataFetcher("hello") { "world" }
+                            .dataFetcher("namespace") { emptyMap<String, Any?>() }
+                    }
+            },
+        ),
+    ),
+) {
+    override fun makeExecutionHints(): NadelExecutionHints.Builder {
+        return super.makeExecutionHints()
+            .newResultMergerAndNamespacedTypename { true }
+    }
+}

test/src/test/kotlin/graphql/nadel/tests/next/fixtures/namespaced/EmptyNamespaceIsEmptyObjectTestSnapshot.kt

@@ -0,0 +1,84 @@
+// @formatter:off
+package graphql.nadel.tests.next.fixtures.namespaced
+
+import graphql.nadel.tests.next.ExpectedNadelResult
+import graphql.nadel.tests.next.ExpectedServiceCall
+import graphql.nadel.tests.next.TestSnapshot
+import graphql.nadel.tests.next.listOfJsonStrings
+import kotlin.Suppress
+import kotlin.collections.List
+import kotlin.collections.listOf
+
+private suspend fun main() {
+    graphql.nadel.tests.next.update<EmptyNamespaceIsEmptyObjectTest>()
+}
+
+/**
+ * This class is generated. Do NOT modify.
+ *
+ * Refer to [graphql.nadel.tests.next.UpdateTestSnapshots]
+ */
+@Suppress("unused")
+public class EmptyNamespaceIsEmptyObjectTestSnapshot : TestSnapshot() {
+    /**
+     * Query
+     *
+     * ```graphql
+     * query {
+     *   hello
+     *   namespace {
+     *     foo @skip(if: true)
+     *   }
+     * }
+     * ```
+     *
+     * Variables
+     *
+     * ```json
+     * {}
+     * ```
+     */
+    override val calls: List<ExpectedServiceCall> = listOf(
+            ExpectedServiceCall(
+                service = "monolith",
+                query = """
+                | {
+                |   hello
+                | }
+                """.trimMargin(),
+                variables = "{}",
+                result = """
+                | {
+                |   "data": {
+                |     "hello": "world"
+                |   }
+                | }
+                """.trimMargin(),
+                delayedResults = listOfJsonStrings(
+                ),
+            ),
+        )
+
+    /**
+     * ```json
+     * {
+     *   "data": {
+     *     "hello": "world",
+     *     "namespace": {}
+     *   }
+     * }
+     * ```
+     */
+    override val result: ExpectedNadelResult = ExpectedNadelResult(
+            result = """
+            | {
+            |   "data": {
+            |     "hello": "world",
+            |     "namespace": {}
+            |   }
+            | }
+            """.trimMargin(),
+            delayedResults = listOfJsonStrings(
+            ),
+        )
+}