When we start a sprint with stories containing sub-tasks, the sprint field value of the sub-task is empty. As a result, if a sub-task is converted to a story, it move out of the sprint, because the sprint value is empty.

Steps to Reproduce:
1. Start a sprint with issue containing sub-tasks.
2. Convert a sub-task to an issue.

Expected Result:
The converted sub-task stays in the sprint

Actual Result:
The converted sub-task moves out of the sprint.