content: from django.db import migrations, models
  
  
  def set_null_values(apps, schema_editor):
      """
      Replace empty strings with null values.
      """
      IKEPolicy = apps.get_model('vpn', 'IKEPolicy')
      IKEProposal = apps.get_model('vpn', 'IKEProposal')
      IPSecProposal = apps.get_model('vpn', 'IPSecProposal')
      db_alias = schema_editor.connection.alias
  
      IKEPolicy.objects.using(db_alias).filter(mode='').update(mode=None)
      IKEProposal.objects.using(db_alias).filter(authentication_algorithm='').update(authentication_algorithm=None)
      IPSecProposal.objects.using(db_alias).filter(authentication_algorithm='').update(authentication_algorithm=None)
      IPSecProposal.objects.using(db_alias).filter(encryption_algorithm='').update(encryption_algorithm=None)
  
  
  class Migration(migrations.Migration):
      dependencies = [
          ('vpn', '0005_rename_indexes'),
      ]
  
      operations = [
          migrations.AlterField(
              model_name='ikepolicy',
              name='mode',
              field=models.CharField(blank=True, null=True),
          ),
          migrations.AlterField(
              model_name='ikeproposal',
              name='authentication_algorithm',
              field=models.CharField(blank=True, null=True),
          ),
          migrations.AlterField(
              model_name='ipsecproposal',
              name='authentication_algorithm',
              field=models.CharField(blank=True, null=True),
          ),
          migrations.AlterField(
              model_name='ipsecproposal',
              name='encryption_algorithm',
              field=models.CharField(blank=True, null=True),
          ),
          migrations.RunPython(code=set_null_values, reverse_code=migrations.RunPython.noop),
      ]
  