(form validation) As a shop owner when I create a plan with only an up-front charge and check 'Recurring Charge' the interval unit should be null

[chrisjsimpson]

Is your feature request related to a problem? Please describe.

If you create a plan and check "Recurring Charge?" but give a zero amount, but do give an up-front chage, the plan is displayed in the shop admin dashboard with an interval unit (e.g. monthly) even though the plan is only a one-off/one-time chage plan whcih is confusing.

Detail: A shop owner may wish to create a Subscription of zero amount charged each interval, but presently Subscribie only supports free plans without an interval (CHECK).

Describe the solution you'd like

Remove/null the interval_unit value of the plan model if the shop owner checks "Recurring Charge?" but leaves interval amount at zero.

Describe alternatives you've considered

Additional context