Fixed an incompatibility with third party inventory management.

[BUGFIX] Fixed two undefined variable warnings introduced in v2.2.
[BUGFIX] Fixed a problem where the newly added option in v2.2 for Purge Products after a Sale broke Magento when the Magento_Inventory module was replaced by third party inventory management. This new option is now only effective when used with the built-in Magento_Inventory module.