Fix: clarify 'Required, can be None' query parameter docs

The documentation incorrectly stated that clients could send None as
a query parameter value. In reality, omitting a required parameter
returns 422, and there is no standard HTTP mechanism to pass None
as a query string value.

Changes:
- Updated docs to clarify that None in the type annotation means
  the OpenAPI schema marks the parameter as nullable
- Added info box explaining the actual behavior
- Fixed xfail tests to verify correct 422 behavior instead of
  expecting impossible 200 responses

Closes #12419

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

@@ -222,10 +222,16 @@ So, when you need to declare a value as required while using `Query`, you can si
 
 ### Required, can be `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's type can be `None`, but that it's still **required** (it must be included in the request). This is useful when you want to require the parameter to be present, but allow the value to be `None`.
 
 To do that, you can declare that `None` is a valid type but simply do not declare a default value:
 
+/// info
+
+When the parameter is required (no default value), omitting it will result in a **422 Validation Error**. The `None` type is included so that the OpenAPI schema correctly represents the parameter as **nullable**, and it allows your code to handle a `None` value explicitly. However, note that there's no standard HTTP mechanism to send `None` as a query parameter value from a client.
+
+///
+
 {* ../../docs_src/query_params_str_validations/tutorial006c_an_py310.py hl[9] *}
 
 ## Query parameter list / multiple values { #query-parameter-list-multiple-values }

tests/test_tutorial/test_query_params_str_validations/test_tutorial006c.py

@@ -22,25 +22,36 @@ 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):
+    """When q is required (no default), omitting it returns 422."""
     response = client.get("/items/")
-    assert response.status_code == 200
+    assert response.status_code == 422
-    assert response.json() == {  # pragma: no cover
+    assert response.json() == {
-        "items": [{"item_id": "Foo"}, {"item_id": "Bar"}],
+        "detail": [
+            {
+                "type": "missing",
+                "loc": ["query", "q"],
+                "msg": "Field required",
+                "input": None,
+            }
+        ]
     }
 
 
-@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):
+    """Empty string fails min_length validation."""
     response = client.get("/items/?q=")
-    assert response.status_code == 200
+    assert response.status_code == 422
-    assert response.json() == {  # pragma: no cover
+    assert response.json() == {
-        "items": [{"item_id": "Foo"}, {"item_id": "Bar"}],
+        "detail": [
+            {
+                "type": "string_too_short",
+                "loc": ["query", "q"],
+                "msg": "String should have at least 3 characters",
+                "input": "",
+                "ctx": {"min_length": 3},
+            }
+        ]
     }