CSV import can create sub-tasks without parents

summary

It's possible to create Issues with the type "Sub-task" that aren't associated with any parent.

steps to reproduce

1. create a CSV source file defining sub-tasks (populate Parent Id and put "Sub-task" as the Issue Type);
2. don't give sub-tasks a unique Issue Id, e.g. use the same string for all of them;
3. perform the import.

Result: Issues are created, but not associated with any parent.

Workaround: this can be fixed by a CSV update. Edit the file to make Issue Id values unique, and reimport the file.