If a task assignee is an expression, and that expression resolves to an empty string, then the task becomes assigned to "". I would like to write this:

${assignee_variable}

Instead, I need to write:

${empty assignee_variable ? null : assignee_variable}

This is surprising, and I can't think of a case where this is the desired behavior. An empty string should work as well as null to leave a task unassigned.