Cannot send test emails before creating subscription, when using non-default filter values

[flamber]

Describe the bug
When quickly testing a subscription by modifying the filters (Enterprise), then it's not possible to send test email.
Regression since 1.41.0 - worked on 1.40.5

Workaround: Save the subscription with modified filter value, then go to the subscription again and click "Send email now".

To Reproduce

1. Add question to a dashboard with a filter - using Count of Products, filter connected to Category with default value Doohickey
   
2. Subscription > Email > change filter value to something different than default, ex Gadget
   
3. Click "Send email now" - it instantly changes to "Sending failed" without any requests being sent, but the browser errors with:

TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'f'
    |     property 'databases' -> object with constructor 'Object'
    |     property '1' -> object with constructor 'l'
    --- property 'metadata' closes the circle
    at JSON.stringify (<anonymous>)
    at api.js:126
    at l (runtime.js:63)
    at Generator._invoke (runtime.js:293)
    at Generator.next (runtime.js:118)
    at p (app-main.bundle.js?bd9515e770dfa46d6cc9:2)
    at a (app-main.bundle.js?bd9515e770dfa46d6cc9:2)
    at app-main.bundle.js?bd9515e770dfa46d6cc9:2
    at new Promise (<anonymous>)
    at Object.<anonymous> (app-main.bundle.js?bd9515e770dfa46d6cc9:2)
(anonymous) @ ActionButton.jsx:103

Information about your Metabase Installation:
Tested 1.40.5 thru 1.41.1 - regression since 1.41.0