enable strict null check for src/vs/base/test/common/async.test.ts (#65442)

src/tsconfig.strictNullChecks.json

@@ -120,6 +120,7 @@
 		"./vs/base/test/browser/ui/tree/objectTreeModel.test.ts",
 		"./vs/base/test/common/arrays.test.ts",
 		"./vs/base/test/common/assert.test.ts",
+		"./vs/base/test/common/async.test.ts",
 		"./vs/base/test/common/cache.test.ts",
 		"./vs/base/test/common/cancellation.test.ts",
 		"./vs/base/test/common/charCode.test.ts",

src/vs/base/test/common/async.test.ts

@@ -522,7 +522,7 @@ suite('Async', () => {
 		assert.ok(r2Queue);
 		assert.equal(r1Queue, queue.queueFor(URI.file('/some/path'))); // same queue returned
 
-		let syncPromiseFactory = () => Promise.resolve(null);
+		let syncPromiseFactory = () => Promise.resolve(undefined);
 
 		r1Queue.queue(syncPromiseFactory);