[dove/cli] Address strictNullCheck error on app.test.ts

[FossPrime]

Strict null checks are on in our tsconfig.json... so we run into the following error without this PR.
https://stackoverflow.com/a/58401023/370238

https://codesandbox.io/s/feathers-dove-cs7qc?file=/test/app.test.ts

Before PR:

test/app.test.ts:36:28 - error TS2532: Object is possibly 'undefined'.

36         assert.strictEqual(response.data.name, 'NotFound')
                              ~~~~~~~~

After PR:

  Feathers application tests
    ✔ starts and shows the index page (44ms)
    ✔ shows a 404 JSON error

  'messages' service
    ✔ registered the service


  3 passing (54ms)

Summary

(If you have not already please refer to the contributing guideline as described
here)

• Tell us about the problem your pull request is solving.
• Are there any open issues that are related to this?
• Is this PR dependent on PRs in other repos?