Remove useless null value.

biscuit/apps/chronos/migrations/0001_initial.py

@@ -90,7 +90,7 @@ class Migration(migrations.Migration):
                 ('room', models.ForeignKey(null=True, blank=True, on_delete=django.db.models.deletion.CASCADE, to='chronos.Room', verbose_name='Room')),
                 ('school', models.ForeignKey(default=biscuit.core.util.core_helpers.get_current_school, on_delete=django.db.models.deletion.CASCADE, to='core.School')),
                 ('subject', models.ForeignKey(null=True, blank=True, on_delete=django.db.models.deletion.CASCADE, related_name='lesson_substitutions', to='chronos.Subject', verbose_name='Subject')),
-                ('teachers', models.ManyToManyField(null=True, blank=True, related_name='lesson_substitutions', to='core.Person')),
+                ('teachers', models.ManyToManyField(blank=True, related_name='lesson_substitutions', to='core.Person')),
             ],
             options={
                 'ordering': ['lesson_period__lesson__date_start', 'week', 'lesson_period__period__weekday', 'lesson_period__period__period'],

biscuit/apps/chronos/models.py

@@ -267,7 +267,7 @@ class LessonSubstitution(SchoolRelated):
         'Subject', on_delete=models.CASCADE,
         related_name='lesson_substitutions', null=True, blank=True, verbose_name=_('Subject'))
     teachers = models.ManyToManyField('core.Person',
-                                      related_name='lesson_substitutions', blank=True, null=True)
+                                      related_name='lesson_substitutions', blank=True)
     room = models.ForeignKey('Room', models.CASCADE, null=True, blank=True, verbose_name=_('Room'))
 
     cancelled = models.BooleanField(default=False)