Challenges: Error when signing up for challenge when mod has set allowed tags to 0 and checked "Fandom only" without selecting a Tag Set

To Replicate:

1. Create a prompt meme or gift exchange challenge

2. In Request Settings, set

   1. all Required and Allowed to 0 for all tag types

   2. Description to allowed (should be pre-selected)

3. In the Tag Options section, set

   1. Tag Sets to Use: blank

   2. Character Settings: check Fandom Only

   3. Relationship Settings: check Fandom Only

4. Submit.

5. Navigate to the Prompt or Sign-up Form and attempt to sign up. Note that you have to actually enter something in the Request section’s “Description” field to trigger the error described in the issue. If you enter nothing, you get a standard, “Sorry! We couldn't save this challenge signup because…” error.

6. Admire the browser-formatted 500 (Chrome 78 on Windows) or blank white page (Safari 13 on Mac), depending on your browser.

What should happen:

It should allow you to save the sign-up, just like it does if you set the Allowed tags to a number greater than 0.