If user entity does not have a user_picture field viewing Task fails with PHP fatal error

Problem/Motivation

When viewing a task, a PHP error is thrown if the user entity does not have a field with machine name `user_picture`

Message 	Error: Call to a member function isEmpty() on null in Drupal\burndown\Entity\Task->getOwnerImage() (line 928 of /home/ide/project/docroot/modules/custom/burndown/src/Entity/Task.php)

#0 /home/ide/project/docroot/modules/custom/burndown/burndown_task.page.inc(45): Drupal\burndown\Entity\Task->getOwnerImage()

Marking as minor priority because not many people actually use the minimal theme so odds of this cropping up should be low.

Steps to reproduce

Install a site using the minimal profile
Enable burndown, set up a project
Add a task
Observe 500 error thrown and message above appearing in log

Proposed resolution

Task::getOwnerImage() needs to check that the `user_picture` field exists before calling isEmpty() on the field.

Remaining tasks

User interface changes

API changes

Data model changes