fix(util): cache nullish values in lazy() (#11564)

lazy() used defaultValue ??= cb(), so a callback returning null or undefined
was never cached and re-ran on every call, breaking the compute-once contract
(any side effect in the callback repeated). Track a called flag instead of
relying on value nullishness.

packages/util/__tests__/lazy.test.ts

@@ -29,4 +29,28 @@ describe('lazy', () => {
 		expect(callback).toHaveBeenCalledOnce();
 		expect(cachedValue).toEqual('Lorem Ipsum');
 	});
+
+	test('GIVEN undefined callback with cached value THEN returns the same', () => {
+		const callback = vi.fn<() => string | undefined>(() => undefined);
+
+		const lazyStoredValue = lazy(callback);
+
+		lazyStoredValue();
+		const cachedValue = lazyStoredValue();
+
+		expect(callback).toHaveBeenCalledOnce();
+		expect(cachedValue).toBeUndefined();
+	});
+
+	test('GIVEN null callback with cached value THEN returns the same', () => {
+		const callback = vi.fn(() => null);
+
+		const lazyStoredValue = lazy(callback);
+
+		lazyStoredValue();
+		const cachedValue = lazyStoredValue();
+
+		expect(callback).toHaveBeenCalledOnce();
+		expect(cachedValue).toBeNull();
+	});
 });

packages/util/src/functions/lazy.ts

@@ -13,6 +13,11 @@
 // eslint-disable-next-line promise/prefer-await-to-callbacks
 export function lazy<Value>(cb: () => Value): () => Value {
 	let defaultValue: Value;
-	// eslint-disable-next-line promise/prefer-await-to-callbacks
-	return () => (defaultValue ??= cb());
+	let called = false;
+	return () => {
+		if (called) return defaultValue;
+		called = true;
+		// eslint-disable-next-line promise/prefer-await-to-callbacks
+		return (defaultValue = cb());
+	};
 }