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.
Merge pull request #42 from robryanx/v160

Updated admin zen_not_null to bring it in line with the non admin function