refactor(handler): response val must be string or null

Follow-up fix from #127

Author: enisdenjo

src/handler.ts

@@ -120,7 +120,7 @@ export type Response = readonly [body: ResponseBody | null, init: ResponseInit];
 function isResponse(val: unknown): val is Response {
   // Make sure the contents of body match string | null
   if (!Array.isArray(val)) return false;
-  if (typeof val[0] !== 'string' || val[0] !== null) return false;
+  if (typeof val[0] !== 'string' && val[0] !== null) return false;
   if (!isObject(val[1])) return false;
 
   // Make sure the contents of init match ResponseInit

tests/handler.test.ts

@@ -8,7 +8,7 @@ it.each(['schema', 'context', 'onSubscribe', 'onOperation'])(
   async (option) => {
     const { request } = createTHandler({
       [option]: () => {
-        return [null, { status: 418 }];
+        return [null, { status: 418, statusText: '' }];
       },
     });