Fix bug with conditions referencing keys outside xPDOQuery::$_class

This problem was introduced in the last commit and will address modxcms/revolution#11203
Merge remote-tracking branch 'xpdo/develop' into develop

* xpdo/develop:
  Fix bug with conditions referencing keys outside xPDOQuery::$_class
Fix bug with conditions referencing keys outside xPDOQuery::$_class

Fixes #11203

Merge remote-tracking branch 'xpdo/develop' into release-2.2

* xpdo/develop:
  Fix bug with conditions referencing keys outside xPDOQuery::$_class