fix returning null from endpoints

A recent change [1] introduced a bug which converts `null` to `{}`
(empty object) when returned from endpoints. This commit explicitly
checks for `undefined` and does not modify `null` responses.

[1]: sveltejs#1808

packages/kit/src/runtime/server/endpoint.js

@@ -63,7 +63,7 @@ export default async function render_route(request, route) {
 				(!type || type.startsWith('application/json'))
 			) {
 				headers = { ...headers, 'content-type': 'application/json; charset=utf-8' };
-				normalized_body = JSON.stringify(body || {});
+				normalized_body = JSON.stringify(typeof body === 'undefined' ? {} : body);
 			} else {
 				normalized_body = /** @type {import('types/hooks').StrictBody} */ (body);
 			}

packages/kit/test/apps/basics/src/routes/endpoint-output/_tests.js

@@ -36,4 +36,10 @@ export default function (test) {
 		const res = await fetch('/endpoint-output/empty');
 		assert.type(await res.buffer(), 'object');
 	});
+
+	test('null body returns null json value', null, async ({ fetch }) => {
+		const res = await fetch('/endpoint-output/null');
+		assert.equal(res.status, 200);
+		assert.equal(await res.json(), null);
+	});
 }

packages/kit/test/apps/basics/src/routes/endpoint-output/null.js

@@ -0,0 +1,4 @@
+/** @type {import('@sveltejs/kit').RequestHandler} */
+export function get() {
+	return { body: null };
+}