fix(SFT-2801): minimum amount value used in Stripe payment intents

[seandelaney]

Fixes #2539

None of the payment intent stuff that handles the amount has changed in about 3 years.

I cannot see anything in the Stripe API docs to say something changed on their end. The docs do say the amount field is required, min value 0.5 USD.

I dunno why its suddenly broken. It previously worked with 0.

I have added a fallback to prevent the initial payment intent from failing on page load.