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:
1. Create new herald rule that assigns to an empty user based on association with a project
2. Create new maniphest task assigned to yourself
3. Associate new task with the project, triggering the herald rule
4. Observe task remains assigned to you
Here's a screenshot of an example:
{F725298}
Then, on the task:
{F725361}
Running `a3393c3ecb92753277cd7bf447631ebb510f59ce`