At the moment, assigning a task to an empty user doesn't place the task up for grabs, as it does when assigning to an empty user from the maniphest task UI.
Steps to reproduce:
- Create new herald rule that assigns to an empty user based on association with a project
- Create new maniphest task assigned to yourself
- Associate new task with the project, triggering the herald rule
- Observe task remains assigned to you
Here's a screenshot of an example:
Then, on the task: