Add test for null response type conversion to void

Verify that OpenAPI 3.1 responses with `type: 'null'` are converted
to void return types instead of generating a broken Null model import.

Author: guizmaii

modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java

@@ -469,6 +469,35 @@ public void testNullableOneOfDoesNotImportNullModel() throws IOException {
         TestUtils.assertFileNotExists(Paths.get(output + "/models/Null.ts"));
     }
 
+    @Test(description = "Verify null response type is converted to void")
+    public void testNullResponseTypeConvertedToVoid() throws IOException {
+        File output = generate(
+            Collections.emptyMap(),
+            "src/test/resources/3_0/typescript-fetch/null-response.yaml"
+        );
+
+        Path apiFile = Paths.get(output + "/apis/ItemsApi.ts");
+        TestUtils.assertFileExists(apiFile);
+
+        // Should not import or reference a "Null" model
+        TestUtils.assertFileNotContains(apiFile, "Null,");
+        TestUtils.assertFileNotContains(apiFile, "NullFromJSON");
+        TestUtils.assertFileNotContains(apiFile, "NullToJSON");
+
+        // Delete endpoint should use void return type
+        TestUtils.assertFileContains(apiFile, "Promise<runtime.ApiResponse<void>>");
+        TestUtils.assertFileContains(apiFile, "VoidApiResponse");
+
+        // Get endpoint should still use Item model
+        TestUtils.assertFileContains(apiFile, "ItemFromJSON");
+        TestUtils.assertFileContains(apiFile, "Promise<runtime.ApiResponse<Item>>");
+
+        // No Null.ts model should be generated
+        TestUtils.assertFileNotExists(Paths.get(output + "/models/Null.ts"));
+        // Item model should still exist
+        TestUtils.assertFileExists(Paths.get(output + "/models/Item.ts"));
+    }
+
     @Test(description = "Verify validationAttributes works with withoutRuntimeChecks=true")
     public void testValidationAttributesWithWithoutRuntimeChecks() throws IOException {
         Map<String, Object> properties = new HashMap<>();

modules/openapi-generator/src/test/resources/3_0/typescript-fetch/null-response.yaml

@@ -0,0 +1,54 @@
+openapi: 3.1.0
+info:
+  version: 1.0.0
+  title: testing null response type conversion to void
+paths:
+  /items/{itemId}:
+    delete:
+      tags:
+        - items
+      operationId: deleteItem
+      summary: Delete an item
+      parameters:
+        - name: itemId
+          in: path
+          required: true
+          schema:
+            type: string
+      responses:
+        '200':
+          description: Item deleted successfully
+          content:
+            application/json:
+              schema:
+                type: 'null'
+    get:
+      tags:
+        - items
+      operationId: getItem
+      summary: Get an item by ID
+      parameters:
+        - name: itemId
+          in: path
+          required: true
+          schema:
+            type: string
+      responses:
+        '200':
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Item'
+components:
+  schemas:
+    Item:
+      type: object
+      properties:
+        id:
+          type: string
+        name:
+          type: string
+      required:
+        - id
+        - name