fix(dashboard): check theme mod key

[miguelpeixe]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

1205919985867982-as-1208686181349205/f

Checks the key to prevent a backend warning and a javascript fatal.

How to test the changes in this Pull Request:

1. While on the epic branch, delete the theme_mods_newspack-theme option from the db
2. Navigate to Newspack -> Dashboard and confirm the page renders blank with a js error
3. Checkout this branch, refresh the page and confirm the dashboard renders without errors

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?