Treat an empty application/json body as {} in parse_form

[linguistic76]

Problem

parse_form runs for every route (via _wrap_req) before handlers, to build the params
dict. It already guards an empty multipart/form-data body (returns FormData()), but the
application/json branch calls await req.json() directly. Starlette's request.json() does
json.loads(b"") on an empty body → JSONDecodeError, which is uncaught → 500.

So a bodyless POST sent with Content-Type: application/json — common from HTTP clients/SDKs
that set that header by default — 500s before reaching the route, even for endpoints that take
no body.

Reproduction

from fasthtml.common import FastHTML
from starlette.testclient import TestClient

app = FastHTML()
@app.post("/x")
async def x(request): return "ok"

TestClient(app, raise_server_exceptions=False).post(
    "/x", headers={"content-type": "application/json"}
)  # 500

File ".../fasthtml/core.py", line ~165, in parse_form
    return await req.json() if ctype == 'application/json' else await req.form()
File ".../starlette/requests.py", line 259, in json
    self._json = json.loads(body)
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

(Starlette intentionally won't tolerate this in request.json() — Kludex/starlette#788 — so the
guard belongs here, alongside the existing empty-multipart one.)

Fix

Mirror the empty-multipart guard for json: an empty json body yields {} (reusing the cached
body), non-empty parses as before. Other content types are unchanged.

    body = await req.body()  # Cache body for non-multipart request types
    if ctype == 'application/json': return await req.json() if body else {}
    return await req.form()

Adds a test in nbs/api/00_core.ipynb asserting an empty application/json POST returns 200
with {} (instead of 500), and that a non-empty json body still parses to its dict (the
existing path is preserved). nbdev_test --file_glob 00_core.ipynb passes.