plan.trial_period_days can be None, yet plan_card.html attempts math on trial_period_days

[chrisjsimpson]

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

plan.trial_period_days can be None, yet plan_card.html attempts math on trial_period_days.

subscribie/themes/theme-jesmond/jesmond/macros/plan_card.html", line 23, in template\n    {% if plan.trial_period_days > 0 %}\n    ^^^^^^^^^^^^^^^^^^^^^^^^^\nTypeError: \'>\' not supported between instances of \'NoneType\' and \'int\

Describe the solution you'd like

Default plan.trial_period_days to 0 and/or update template to be more robust.

Describe alternatives you've considered

Additional context
Consider both existing and new shops data modem / data migrations