fix: add undefined check for expr in parseItem function

[ShawnAtActivate]

This isn't a "bug" with the code but rather uncovered by bad / unsupported functionality when using laravels __ function in unintended ways

• Prevents errors when expr is null or undefined allowing build to succeed.
• Follow same path as if expr is null

[ShawnAtActivate]

I did some further digging in to what causes this bug. It's due to a php lang file on my end trying to make a laravel localization call to another localization file

<?php

return array(
  'test' => __('anotherfile')
);

The crash happens when trying to parseItem(expr.value) if value is not set of undefined because of bad data resulting in an undefined type and the build error displayed in the PR

TL;DR

• Tried to nest lang translations using __
• Found crash where parseItem checks for .value on a callback function that doesnt exist resulting in undefined being passed
• Boom