Nullable ForeignKey field does not get updated to null

Describe the bug
Nullable ForeignKey field does not get updated to null

To Reproduce
Steps to reproduce the behavior:

Create 2 models, User and Vehicle, User model will have a vehicle foreignKey field which is nullable

class Vehicle(ormar.Model):
    class Meta(BaseMeta):
        tablename = "vehicles"

    id: int = ormar.Integer(primary_key=True)
    chasis_no: str = ormar.String(max_length=64, unique=True)
    registration_no: str = ormar.String(max_length=64, unique=True)
    model: str = ormar.String(max_length=128)


class User(ormar.Model):
    class Meta(BaseMeta):
        tablename = "users"

    id: int = ormar.Integer(primary_key=True)
    name: str = ormar.String(max_length=64, unique=True)
    vehicle: Vehicle = ormar.ForeignKey(to=Vehicle, nullable=True)
    email: str = ormar.String(nullable=True, max_length=100)

Create a Vehicle (via an endpoint exposed)

@app.post("/vehicle")
async def add_vehicle():
    vehicle = await Vehicle.objects.create(
        chasis_no="12345",
        registration_no="56789",
        model="Hilux"
    )
    return vehicle

Create User with vehicle from above step, User gets created fine with vehicle (via an endpoint exposed)

@app.post("/user")
async def add_user(request: AddUserRequest):
    vehicle = await Vehicle.objects.get_or_none(id=request.vehicle_id)
    user = await User.objects.create(
        name=request.name,
        vehicle=vehicle,
        email=request.email
    )
    return user

Update User created above, set vehicle=None and email=None (via an endpoint exposed)

@app.post("/del_vehicle_email_from_user")
async def unassign_vehicle_from_user(request: UnassignVehicleRequest):
    user = await User.objects.get_or_none(id=request.user_id)
    user.vehicle = None
    user.email = None
    user = await user.update(_columns=["vehicle", "email"])
    return user

Open Users table in database and check, Vehicle column which is a foreign-key wont be updated to null but email gets updated to null

Full code of main.py

import uvicorn
from fastapi import FastAPI
from pydantic.main import BaseModel

from db import User, Vehicle, database

app = FastAPI()


@app.on_event("startup")
async def startup():
    if not database.is_connected:
        await database.connect()


@app.on_event("shutdown")
async def shutdown():
    if database.is_connected:
        await database.disconnect()


class AddUserRequest(BaseModel):
    vehicle_id: int
    name: str
    email: str


class UnassignVehicleRequest(BaseModel):
    user_id: int


@app.post("/vehicle")
async def add_vehicle():
    vehicle = await Vehicle.objects.create(
        chasis_no="12345",
        registration_no="56789",
        model="Hilux"
    )
    return vehicle


@app.post("/user")
async def add_user(request: AddUserRequest):
    vehicle = await Vehicle.objects.get_or_none(id=request.vehicle_id)
    user = await User.objects.create(
        name=request.name,
        vehicle=vehicle,
        email=request.email
    )
    return user


@app.post("/del_vehicle_email_from_user")
async def unassign_vehicle_from_user(request: UnassignVehicleRequest):
    user = await User.objects.get_or_none(id=request.user_id)
    user.vehicle = None
    user.email = None
    user = await user.update(_columns=["vehicle", "email"])
    return user


if __name__ == "__main__":
    uvicorn.run(app=app, port=8080, host="127.0.0.1", reload=False)

Expected behavior
Nullable foreignkey field should be set to null when updating

Screenshots
Added above

Versions (please complete the following information):

Database backend used - postgres
Python version - 3.11
ormar- 0.12.1
pydantic- 1.10.4
fastapi - 0.104.1

Additional context
Creating a User with vehicle=None works fine(Issue arises only while updating to None)