Is the setting "disable unassigned issues" in your JIRA's general configuration switched on? If not, then the project lead is used as a fallback, otherwise you should be able to create subtasks without an assignee set.
More documentation about how to set the assignee can be found in the wiki:
If you've checked the settings and think that it is an error, please feel free to file a new feature/bug request in the project's issue tracker here:
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