Fix Django BookInstance.book model to have null default value (#2798)

files/en-us/learn/server-side/django/models/index.html

@@ -338,7 +338,7 @@ <h3 id="BookInstance_model">BookInstance model</h3>
 class BookInstance(models.Model):
     """Model representing a specific copy of a book (i.e. that can be borrowed from the library)."""
     id = models.UUIDField(primary_key=True, default=uuid.uuid4, help_text='Unique ID for this particular book across whole library')
-    book = models.ForeignKey('Book', on_delete=models.RESTRICT)
+    book = models.ForeignKey('Book', on_delete=models.RESTRICT, null=True)
     imprint = models.CharField(max_length=200)
     due_back = models.DateField(null=True, blank=True)