Updated admin zen_not_null to bring it in line with the non admin function.
The primary concern with the admin one was zen_not_null(true) === false
The website one correctly does zen_not_null(true) === true
An explicit check for boolean could also be added but I guess it doesn't need it.
CHANGE-688 Shipping needs a way to adjust order total used in quotes
Adjust for taxes
CHANGE-491 - Updating timezone-detection error help message in zc_install
CHANGE-18 - Installer - update version-detection rules
ISSUE-83 - lat9 requested more notifiers for order-class
ISSUE-84 Request Class

see https://confluence.zen-cart.com/display/PUBDOC/Request+Class
ISSUE-87 - Fix payment module problem admin-side preventing use of Refund option
ISSUE-88 - Fix var assignment operator in ot_gv.php for Calculate Tax
CHANGE-694 - Stopped admin send-mail page from drawing a huge dropdown list even when a single customer is pre-selected from customers screen
ISSUE-86 - fixed comment
ISSUE-84 - Fixed sidebox references to $GET[main_page] to $current_page_base instead, to accommodate implementation of Request Class
ISSUE-90 - Add gTLD support for email addresses (like .marketing or .international)
ISSUE-84 - Fixing errors triggered by request class unit-tests
CHANGE-695 Coupons pagination has issues with rounding attempt
CHANGE-696 Display of Product Categories is unclear and needs better layout
CHANGE-697 - Change core config entries to not use config-group-id 0 since many sloppy plugin authors delete those core settings
CHANGE-698 - Fix bugs in calls to zenCssButton()
ISSUE-89 - Update zenCssButton function and stylesheet to use CSS3 (courtesy of lat9 contribution)
Merge pull request #42 from robryanx/v160

Updated admin zen_not_null to bring it in line with the non admin function
CHANGE-18 - Installer - update version-detection rules to test for buggy CGI versions of PHP