docs: clarify required but nullable request body fields

docs/en/docs/tutorial/body.md

@@ -55,6 +55,36 @@ For example, this model above declares a JSON "`object`" (or Python `dict`) like
 }
 ```
 
+### Required fields that can be `None`
+
+In Python type hints, a parameter can be **required** and still allow the value `None`.
+
+This means that the field must be present in the request body, but its value can be `null`
+(`None` in Python).
+
+This typically happens when you use `Optional[T]` **without** providing a default value.
+
+For example:
+
+{* ../../docs_src/body/tutorial005_py39.py hl[6] *}
+
+And for Python 3.10+:
+
+{* ../../docs_src/body/tutorial005_py310.py hl[6] *}
+
+In this example:
+
+* The `description` field is **required** (the client must include it in the JSON body).
+* Its value can still be `null` (`None` in Python).
+* This is different from a truly optional field, which would have a default value:
+
+```python
+class Item(BaseModel):
+    description: Optional[str] = None
+```
+
+Here, `description` can be omitted entirely in the request body.
+
 ## Declare it as a parameter { #declare-it-as-a-parameter }
 
 To add it to your *path operation*, declare it the same way you declared path and query parameters:

docs_src/body/tutorial005_py310.py

@@ -0,0 +1,11 @@
+from fastapi import FastAPI
+from pydantic import BaseModel
+
+app = FastAPI()
+
+class Item(BaseModel):
+    description: str | None
+
+@app.post("/items/")
+async def create_item(item: Item):
+    return item

docs_src/body/tutorial005_py39.py

@@ -0,0 +1,12 @@
+from typing import Optional
+from fastapi import FastAPI
+from pydantic import BaseModel
+
+app = FastAPI()
+
+class Item(BaseModel):
+    description: Optional[str]
+
+@app.post("/items/")
+async def create_item(item: Item):
+    return item

tests/test_tutorial/test_body_required_nullable/test_tutorial001.py

@@ -0,0 +1,28 @@
+import importlib
+import pytest
+from fastapi.testclient import TestClient
+
+from ...utils import needs_py310
+
+@pytest.fixture(
+    name="client",
+    params=[
+        pytest.param("tutorial005_py39"),
+        pytest.param("tutorial005_py310", marks=needs_py310),
+    ],
+)
+def get_client(request: pytest.FixtureRequest):
+    mod = importlib.import_module(f"docs_src.body.{request.param}")
+    client = TestClient(mod.app)
+    return client
+
+
+def test_required_nullable_field(client: TestClient):
+    response = client.post("/items/", json={"description": None})
+    assert response.status_code == 200
+    assert response.json() == {"description": None}
+
+
+def test_required_field_missing(client: TestClient):
+    response = client.post("/items/", json={})
+    assert response.status_code == 422