fix: 🐛 prenormalize findAll responses in application adapter

Ember Data fetch-manager will throw an error if an array resposne is an
empty object before it reaches the serializer.  Thus we need to
intercept the response in the adapter and "prenormalize" it to contain
an empty items array `{items: []}`.

addons/api/tests/unit/adapters/application-test.js

@@ -3,9 +3,12 @@ import { setupTest } from 'ember-qunit';
 import config from 'ember-get-config';
 import RESTAdapter from '@ember-data/adapter/rest';
 import { InvalidError } from '@ember-data/adapter/error';
+import setupMirage from 'ember-cli-mirage/test-support/setup-mirage';
+import { Response } from 'miragejs';
 
 module('Unit | Adapter | application', function (hooks) {
   setupTest(hooks);
+  setupMirage(hooks);
 
   test('its namespace is equal to the configured namespace', function (assert) {
     assert.expect(2);
@@ -24,12 +27,23 @@ module('Unit | Adapter | application', function (hooks) {
     assert.expect(1);
     const adapter = this.owner.lookup('adapter:application');
     // TODO this is icky, should be changed to a spy or stub
+    const originalAjax = RESTAdapter.prototype.ajax;
     RESTAdapter.prototype.ajax = (url, type) => {
       assert.equal(type, 'PATCH');
+      RESTAdapter.prototype.ajax = originalAjax;
     };
     adapter.ajax('/', 'PUT');
   });
 
+  test('it prenormalizes "empty" responses into a form that the fetch-manager will not reject', async function (assert) {
+    assert.expect(1);
+    const adapter = this.owner.lookup('adapter:project');
+    const store = this.owner.lookup('service:store');
+    this.server.get('/v1/projects', () => new Response({}));
+    const prenormalized = await adapter.findAll(store, {modelName: 'project'}, null, []);
+    assert.deepEqual(prenormalized, {items: []});
+  });
+
   test('it correctly identifies 400 responses as invalid', function (assert) {
     assert.expect(2);
     const adapter = this.owner.lookup('adapter:application');