[BUG] save() results in OperationFailure if use_revision=True and keep_nulls=False

[poccio]

Describe the bug
Defining a document with use_revision=True and keep_nulls=False results in .save() raising OperationFailure when invoked on new documents. I am using beanie==1.26.0.

To Reproduce

import asyncio
import os

from beanie import Document, init_beanie
from motor.motor_asyncio import AsyncIOMotorClient


class Parent(Document):
    name: str

    class Settings:
        name = "parents"
        use_revision = True
        keep_nulls = False


async def main():
    client = AsyncIOMotorClient(os.environ["MONGO_URI"])
    await init_beanie(database=client.test, document_models=[Parent])

    # raises OperationFailure
    parent = Parent(name="P1")
    await parent.save()

    # works
    parent = Parent(name="P2")
    await parent.insert()


if __name__ == "__main__":
    asyncio.run(main())

Expected behavior
I would expect .save() to successfully insert the new document, as stated in the docs.

Additional context
I did some debugging and - I may be wrong but - the issue seems to be with the update query:

{
    "$set": {
        ..., 
        "revision_id": Binary(...)
    }, 
    "$unset": {
        "revision_id": None
    }
}

which tells Mongo to both set revision_id to a value and unset it. This is a result of this Unset operator, which specifies to unset revision_id (as it is, at that point, a top-level None).

(First issue here, thanks for the amazing library)