Migrator converts NULL values to a string "None"

[sommelon]

Hi.
I am using SQLite 3 and I have this model

class AuthConfig(models.Model):
   username = fields.EncryptedCharField(max_length=150, null=True, blank=True, verbose_name=_("Username"))
   password = fields.EncryptedCharField(null=True, blank=True, max_length=150, verbose_name=_("Password"))
   token = fields.EncryptedCharField(max_length=1500, null=True, blank=True, verbose_name=_("Token"))

My migration looks like this

migrations.RunPython(
    Migrator("integrations", "AuthConfig", "username").encrypt, reverse_code=Migrator("integrations", 'AuthConfig', 'username').decrypt
        ),
migrations.RunPython(
    Migrator("integrations", "AuthConfig", "password").encrypt, reverse_code=Migrator("integrations", 'AuthConfig', 'password').decrypt
        ),
migrations.RunPython(
    Migrator("integrations", "AuthConfig", "token").encrypt, reverse_code=Migrator("integrations", 'AuthConfig', 'token').decrypt
),

When the original field value is saved in the database as NULL, applying the migration will change the value to a string "None" because of the string interpolation used to construct an UPDATE statement in Migrators execute() method.

To reproduce:

1. Make a model with an optional field (null=True)
2. Create a record with that field being NULL
3. Convert the field to an Encrypted field
4. Run the migration with Migrator

[tcitry]

@sommelon Much thanks for the PR, I have upload to pypi with 1.1.8 version.