OperationalError: near "None": syntax error during migrate (SQLite)

### Description
I followed the setup instructions in the README for the django-ledger-starter template, but `python manage.py migrate` fails with an SQLite syntax error near `None`.

### Steps to Reproduce
1. Clone the starter repo:
```bash
git clone https://github.com/arrobalytics/django-ledger-starter.git
cd django-ledger-starter
```
2. Install dependencies:
```bash
pip install pipenv
pipenv install && pipenv shell
```
3. Run migrations:
```bash
python manage.py migrate
```

### Expected Behavior
All migrations should apply successfully and create the necessary tables.

### Actual Behavior
Migration fails at `django_ledger.0026_stagedtransactionmodel_customer_model_and_more` with:

```
django.db.utils.OperationalError: near "None": syntax error
```

### Full Traceback (truncated)
```
Applying django_ledger.0026_stagedtransactionmodel_customer_model_and_more...Traceback (most recent call last):
...
sqlite3.OperationalError: near "None": syntax error
```

### Environment
- OS: Windows 10
- Python: 3.11 (Anaconda)
- Django: Installed via pipenv (version from lockfile)
- Database: SQLite (default from starter project)

### Notes
- Warning also appears before migrations:
```
UserWarning: You are using the deprecated behavior of django_ledger.
```
- Error might be caused by a field default value or constraint in migration 0026 that renders invalid SQL in SQLite.

### Possible Fix
Consider checking migration 0026 in `django_ledger/migrations` — it may contain a field with `default=None` or invalid SQL literal.

Start by reproducing the failure with `python manage.py migrate`, then inspect `django_ledger/migrations/0026_stagedtransactionmodel_customer_model_and_more.py`, especially fields, defaults, and constraints that may render `None` in SQLite SQL. Done means the migration completes successfully with the starter project's default SQLite database and creates the expected tables.

Written by the indexing model from the issue text.