[BUG] Error occurs when saving to a model with the keep_nulls = False setting

[eac0de]

Describe the bug
When I call save() from document with the keep_nulls = False setting:
pymongo.errors.OperationFailure: '$unset' is empty. You must specify a field like so: {$unset: {<field>: ...}}, full error: {'ok': 0.0, 'errmsg': "'$unset' is empty. You must specify a field like so: {$unset: {<field>: ...}}", 'code': 9, 'codeName': 'FailedToParse'}
This is happening because func get_top_level_nones return a empty dictionary when None-values is missing.
[{'$set': {'request_id': ...}}, {'$unset': {}}]

Expected result
The document with the keep_nulls = False setting to save successfully even if there are no None-values

My solution

in method save from Document in branch if self.get_settings().keep_nulls is False:

instead

        if self.get_settings().keep_nulls is False:
            return await self.update(
                SetOperator(
                    get_dict(
                        self,
                        to_db=True,
                        keep_nulls=self.get_settings().keep_nulls,
                    )
                ),
                Unset(get_top_level_nones(self)),
                session=session,
                ignore_revision=ignore_revision,
                upsert=True,
                **kwargs,
            )

do this

        if self.get_settings().keep_nulls is False:
            arguments  = [
                SetOperator(
                    get_dict(
                        self,
                        to_db=True,
                        keep_nulls=self.get_settings().keep_nulls,
                    )
                )
            ]
            nones = get_top_level_nones(self)
            if nones:
                arguments.append(
                    Unset(nones)
                )
            return await self.update(
                *arguments,
                session=session,
                ignore_revision=ignore_revision,
                upsert=True,
                **kwargs,
            )

[github-actions]

This issue is stale because it has been open 30 days with no activity.

[eac0de]

as a temporary solution - add a field with None to the model

class Exemple(Document):
      a: int
      for_beanie_err: None = Field(
        default=None,
        exclude=True,
    )