How to pass null/None?

Nobody has claimed this yet.

I'm unsure where responsibility for this lies; flask-smorest, webargs, openapi, etc. But I cannot make a field nullable. i.e. I want to pass a value to the API so that I receive `None` to SQLAlchemy, or pass nothing and have the field not passed to `kwargs` at all.

Here's the trimmed down code
```python

bp = WrappedBlueprint("Project", __name__, url_prefix="/api/projects")
# ma is a flask_marshmallow.Marshmallow object

class ProjectSchema(ma.SQLAlchemyAutoSchema)
class Meta:
model = Project
projectId = ma.auto_field(required=True)
clientId = ma.auto_field(allow_none=True)

@bp.route("/")
class ProjectResource(MethodView):
@bp.response(HTTPStatus.OK, ProjectSchema())
@bp.arguments(ProjectSchema(only=("clientId",), partial=True), location="query", as_kwargs=True)
@bp.arguments(ProjectSchema(only=("projectId",)), location="path", as_kwargs=True)
def patch(self, **kwargs):
# I want kwargs = {"clientId": None} if None was passed
# or not be present at all if nothing was passed
pass
```

If I perform a patch request via python using requests `requests.patch("http://127.0.0.1:7524/api/projects/125", params={"clientId": None})` the `clientId` never makes it into the kwargs passed to the MethodView's patch function.

I'm at a loss as to how to get the functionality I need for something that I thought would be trivial.

This may or may not be useful, but the openapi.json that is generated does indeed specify `nullable: true`
```
...
"patch": {
"parameters":
[
...
{
"in": "query",
"name": "clientId",
"required": false,
"description": "The ID of the client this project is associated with",
"schema":
{
"type": "integer",
"nullable": true
}
}
]
}
```
However the resulting apidocs page refuses to accept `null`, `None`, `"null"`, etc.

Start by reproducing the PATCH request against the MethodView.patch entry point using the shown ProjectSchema and query parameters. Trace how the clientId query value moves through the bp.arguments decorators and compare it with the generated openapi.json and apidocs behavior. Done means distinguishing an explicitly supplied null value from an omitted clientId in kwargs.

Written by the indexing model from the issue text.