old_contents: from django.db import models
  from oauth2client.django_orm import CredentialsField
  
  
  class Credentials(models.Model):
      client_id = models.CharField(
          max_length=128
      )
      credentials = CredentialsField(
          editable=False
      )
  
new_contents: from django.db import models
  from oauth2client.django_orm import CredentialsField
  
  
  class Credentials(models.Model):
      client_id = models.CharField(
          max_length=128
      )
      credentials = CredentialsField(
          editable=False,
          blank=True,
          null=True,
      )
  
subject: allow for empty creds