docs: finalize Required, can be None example with test verification

docs_src/query_params_str_validations/tutorial006d_an_py310.py

@@ -1,12 +1,11 @@
-from typing import Annotated
-
 from fastapi import FastAPI, Query
+from typing import Annotated, Optional
 
 app = FastAPI()
 
 
 @app.get("/items/")
-async def read_items(q: Annotated[str | None, Query()] = ...):
+async def read_items(q: Annotated[Optional[str], Query()] = ...):
     if q in ("None", "", "null"):
         q = None
     return {"q": q}