@oneOf inputs do not support nullability

[DevJake]

When an input class is marked as @oneOf and has a nullable field, it is not possible to actually provide a null value in the query, and the query completely fails.

Describe the Bug

This bug relates to the usage of input classes that implement the @oneOf directive using one_of=True:

@strawberry.input(one_of=True)
class FilterInput:
    name: str | None

As shown, it should be possible to pass a null value in the query for the name field. However, when executing this query:

query MyQuery {
  test(filterInput: {name: null})
}

You get this response:

{
  "data": null,
  "errors": [
    {
      "message": "Field 'FilterInput.name' must be non-null.",
      "locations": [
        {
          "line": 2,
          "column": 21
        }
      ]
    }
  ]
}

I would assume that the one_of parameter directs Strawberry to evaluate if any of the given fields are None and fail if so, even though this is a legitimate input value.

---

I have provided this minimum working example, which runs using FastAPI. I have also included Poetry's pyproject.toml file so this issue can be replicated with ease:

[tool.poetry]
name = "strawberry-testing"
version = "0.1.0"
description = ""
authors = []

[tool.poetry.dependencies]
python = ">=3.11,<3.12"
strawberry-graphql = "0.260.2"
fastapi = "0.115.8"
uvicorn = "0.34.0"


[build-system]
requires = ["poetry-core"]
build-backend = "poetry.core.masonry.api"

from fastapi import FastAPI
from strawberry.fastapi import GraphQLRouter
import strawberry


@strawberry.input(one_of=True)
class FilterInput:
    name: str | None


@strawberry.input
class Query:
    @strawberry.field
    async def test(self, filter_input: FilterInput) -> str | None:
        return filter_input.name


app = FastAPI()

schema = strawberry.Schema(query=Query)
graphql_app = GraphQLRouter(schema=schema)

app.include_router(graphql_app, prefix="/graphql")