Cannot change default membership period back to 'no end date'

[Dianne Tennent]

MySQL8 on main branch
May be related to this bug here: https://bugs.launchpad.net/mahara/+bug/1965715

To replicate:
1. Create an institution with a default membership period e.g. 3 weeks
2. Save settings
3. Change default membership period to 'No end date'
4. Save settings

Expected:
 - Settings saved

Actual:
 - If I clear the input field and change the option to "No end date", I don't get an error, but the settings are not saved
 - If I just clear the input field and save the settings, I get an error (and settings are not saved):
[WAR] f1 (lib/pieforms/pieform/elements/expiry.php:130) A non-numeric value encountered
Call stack (most recent first):

    log_message("A non-numeric value encountered", 8, true, true, "/home/diannetennent/code/mahara/htdocs/lib/pieform...", 130) at /home/diannetennent/code/mahara/htdocs/lib/errors.php:515
    error(2, "A non-numeric value encountered", "/home/diannetennent/code/mahara/htdocs/lib/pieform...", 130, array(size 7)) at /home/diannetennent/code/mahara/htdocs/lib/pieforms/pieform/elements/expiry.php:130
    pieform_element_expiry_get_value(object(Pieform), array(size 7)) at /home/diannetennent/code/mahara/htdocs/lib/pieforms/pieform.php:928
    Pieform->get_value(array(size 7)) at /home/diannetennent/code/mahara/htdocs/lib/pieforms/pieform.php:1480
    Pieform->get_submitted_values() at /home/diannetennent/code/mahara/htdocs/lib/pieforms/pieform.php:553

Notes:
My instinct in this scenario was to clear the field i.e. remove the number 3 and save, so perhaps this would be the best option, rather than requiring people to also select 'No end date' after they have cleared the field.
    Pieform->__construct(array(size 5)) at /home/diannetennent/code/mahara/htdocs/lib/pieforms/pieform.php:173
    Pieform::process(array(size 5)) at /home/diannetennent/code/mahara/htdocs/lib/mahara.php:5754
    pieform(array(size 5)) at /home/diannetennent/code/mahara/htdocs/admin/users/institutions.php:723