Wrong `null=True` migration behaviour

[Olegt0rr]

aerich didn't see adding null=True to the field

It seems fix for #135 added a new bug

How to reproduce

1. Create model and migrations

class Question(Model):
    text = TextField()

2. Set field as optional

class Question(Model):
    text = TextField(null=True)

3. Create migration

Result:

-- upgrade --
ALTER TABLE "question" ALTER COLUMN "text" TYPE TEXT USING "text"::TEXT;
-- downgrade --
ALTER TABLE "question" ALTER COLUMN "text" TYPE TEXT USING "text"::TEXT;

Expected result:

-- upgrade --
ALTER TABLE "question" ALTER COLUMN "text" DROP NOT NULL;
-- downgrade --
ALTER TABLE "question" ALTER COLUMN "text" SET NOT NULL;

[long2ice]

Fixed