Why is Django swearing at the {{ url }} field?

Question

Django

Hello.
Now I am inserting the following template into the template:

<img class="" src="{{ courseitem.author_image.url }}"/>

If I didn't upload this file, I get an error: The 'author_image' attribute has no file associated with it. and screen of death.
Model goes:

author_image = models.ImageField(upload_to='', blank=True, null=True, help_text="Размер изображения 100 * 100", verbose_name="Фотография автора")

Questions:
1) Why does this template require the file to be required?
2) How to make the template not crash the page if there is no file?