Fix 500 on enabling plugins

[yakkomajuri]

Changes

Please describe.
If this affects the frontend, include screenshots.

Updating the plugin config didn't always send a new config, sometimes it might send something like:

{enabled: true, order: 1}

Only affects plugins with secret fields.

Checklist

• All querysets/queries filter by Organization, by Team, and by User
• Django backend tests
• Jest frontend tests
• Cypress end-to-end tests