PATCH silently ignores a request to clear all tags

A user who wants to remove every tag from a note (e.g. PATCH {"tags": []}) gets a 200 OK response... and their tags are silently left unchanged. There's no error, no indication anything went wrong -- the API just lies about having applied the update. That's worse than a visible failure: the user believes the note is untagged, a client app's local cache shows it untagged, but the server disagrees. This class of bug (silent no-op on a valid, explicit request) is exactly the kind that produces confusing bug reports days later ("I cleared this ages ago, why is it still tagged?") instead of an immediate, traceable failure.

PATCH /notes/{id} is documented as: "Only fields present in the payload are changed." Sending {"tags": []} means "I am explicitly setting tags to be empty," and it should be honored exactly like sending any other explicit value for tags. Instead, the empty list is silently treated as if the field weren't sent at all.

cd fastapi/notes_api source .venv/bin/activate uvicorn app.main:app --reload & ID=$(curl -s -X POST http://127.0.0.1:8000/notes -H "Content-Type: application/json"
-d '{"title":"Groceries","content":"milk","tags":["work","urgent"]}' | python3 -c 'import json,sys;print(json.load(sys.stdin)["id"])') curl -s -X PATCH "http://127.0.0.1:8000/notes/$ID" -H "Content-Type: application/json"
-d '{"tags":[]}'

-> tags is still ["work","urgent"], not []

update_note builds data = payload.model_dump(exclude_unset=True), which correctly distinguishes "field not sent" from "field sent as an empty/falsy value" -- exclude_unset=True is precisely the tool for that distinction, and it's already being used correctly. The bug is in how the tags branch then reads data: it checks truthiness of the value instead of checking whether the key is present, throwing away the information model_dump just gave it.

Compare how the tags branch in update_note checks data against how the title and content branches immediately above it check the same data dict. title/content can't actually be empty strings (both have min_length=1), which is exactly why this class of bug doesn't show up for them -- but tags genuinely can be an empty, still-meaningfully-sent list.