if $dateString is 'null' make no attempt to format it

Membership Receipt for LifeTime Membership - Send Confirmation and Receipts -> from Backend shows:

Membership End Date 0th, 0

Two options:

1) Don't try format if $membership->end_date is 'null'

in CRM_Member_Form_Membership:
if ($membership->end_date !='null')

{ $form->assign('mem_end_date', CRM_Utils_Date::customFormat($membership->end_date, '%B %E%f, %Y')); }

2) Never try to format any $dateString that is 'null':

in CRM_Utils_Date:
if ($dateString == 'null')

{ return ''; }