When importing a sub-task via CSV, you should require a reference to the parent issue.
If you try to import the CSV via Issues > Import Issues from CSV, that validation is done, however when importing through Administration > System > External System Import the CSV import creates orphan sub-tasks.

Steps to reproduce

1. Create a CSV with only a sub-task to be created, with no parent issue specified
2. Import CSV using External System Import

Expected Behaviour
A warning message saying "Subtask 'Issue' doesn't have a valid Parent Id selection. Issue will not be created." should be displayed (as it is on "Import Issues from CSV")

Actual behaviour
Issue is created successfully without a parent