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/package.json

@@ -35,6 +35,7 @@
     "ember-cli-dependency-checker": "^3.2.0",
     "ember-cli-eslint": "^5.1.0",
     "ember-cli-inject-live-reload": "^2.0.2",
+    "ember-cli-mirage": "^1.1.7",
     "ember-cli-sri": "^2.1.1",
     "ember-cli-template-lint": "^1.0.0-beta.3",
     "ember-cli-uglify": "^3.0.0",