Fix #12419: Update "Required, can be None" docs and fix example code

The documentation section "Required, can be None" incorrectly claimed that
a query parameter without a default value can accept None. In practice,
query parameters cannot send None via HTTP, and omitting a required
parameter results in a 422 error.

Changes:
- docs: Reword section to "Optional with None", clarify that = None
  makes the parameter optional (defaults to None)
- tutorial006c_an_py310.py: Add = None default
- tutorial006c_py310.py: Add default=None to Query()
- test_tutorial006c.py: Remove xfail markers, update expected
  required: True → False in OpenAPI schema

docs/en/docs/tutorial/query-params-str-validations.md

@@ -220,14 +220,20 @@ So, when you need to declare a value as required while using `Query`, you can si
 
 {* ../../docs_src/query_params_str_validations/tutorial006_an_py310.py hl[9] *}
 
-### Required, can be `None` { #required-can-be-none }
+### Optional with `None` { #required-can-be-none }
 
-You can declare that a parameter can accept `None`, but that it's still required. This would force clients to send a value, even if the value is `None`.
+You can declare that a parameter is optional and can accept `None`, even when using `Query` for validations.
 
-To do that, you can declare that `None` is a valid type but simply do not declare a default value:
+To do that, you can declare that `None` is a valid type and use `None` as the default value:
 
 {* ../../docs_src/query_params_str_validations/tutorial006c_an_py310.py hl[9] *}
 
+/// tip
+
+This is the same as using `Query(None, min_length=3)`, the parameter is optional and will have a default value of `None`.
+
+///
+
 ## Query parameter list / multiple values { #query-parameter-list-multiple-values }
 
 When you define a query parameter explicitly with `Query` you can also declare it to receive a list of values, or said in another way, to receive multiple values.

docs_src/query_params_str_validations/tutorial006c_an_py310.py

@@ -6,7 +6,7 @@ app = FastAPI()
 
 
 @app.get("/items/")
-async def read_items(q: Annotated[str | None, Query(min_length=3)]):
+async def read_items(q: Annotated[str | None, Query(min_length=3)] = None):
     results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
     if q:
         results.update({"q": q})

docs_src/query_params_str_validations/tutorial006c_py310.py

@@ -4,7 +4,7 @@ app = FastAPI()
 
 
 @app.get("/items/")
-async def read_items(q: str | None = Query(min_length=3)):
+async def read_items(q: str | None = Query(min_length=3, default=None)):
     results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
     if q:
         results.update({"q": q})

tests/test_tutorial/test_query_params_str_validations/test_tutorial006c.py

@@ -22,24 +22,18 @@ def get_client(request: pytest.FixtureRequest):
     return client
 
 
-@pytest.mark.xfail(
-    reason="Code example is not valid. See https://github.com/fastapi/fastapi/issues/12419"
-)
 def test_query_params_str_validations_no_query(client: TestClient):
     response = client.get("/items/")
     assert response.status_code == 200
-    assert response.json() == {  # pragma: no cover
+    assert response.json() == {
         "items": [{"item_id": "Foo"}, {"item_id": "Bar"}],
     }
 
 
-@pytest.mark.xfail(
-    reason="Code example is not valid. See https://github.com/fastapi/fastapi/issues/12419"
-)
 def test_query_params_str_validations_empty_str(client: TestClient):
     response = client.get("/items/?q=")
     assert response.status_code == 200
-    assert response.json() == {  # pragma: no cover
+    assert response.json() == {
         "items": [{"item_id": "Foo"}, {"item_id": "Bar"}],
     }
 
@@ -99,7 +93,7 @@ def test_openapi_schema(client: TestClient):
                         "operationId": "read_items_items__get",
                         "parameters": [
                             {
-                                "required": True,
+                                "required": False,
                                 "schema": {
                                     "anyOf": [
                                         {"type": "string", "minLength": 3},