500 on empty body

[AstraLuma]

if there's a Content-Type that's recognized and the body is empty, then an error is raised instead of a 400.

  File "/usr/local/lib/python3.13/site-packages/starlette/routing.py", line 73, in app
    response = await f(request)
               ^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/starlette_wtf/csrf.py", line 175, in endpoint_wrapper
    signed_token = await get_csrf_token(request)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/starlette_wtf/csrf.py", line 246, in get_csrf_token
    formdata = await get_formdata(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/starlette_wtf/util.py", line 59, in get_formdata
    formdata = ImmutableMultiDict(await request.json())
                                  ^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/starlette/requests.py", line 249, in json
    self._json = json.loads(body)
                 ~~~~~~~~~~^^^^^^
  File "/usr/local/lib/python3.13/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^
  File "/usr/local/lib/python3.13/json/decoder.py", line 345, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/json/decoder.py", line 363, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

[amorey]

I created a PR to address this:
#47

The problem is that currently JSON decode errors are being bubbled up to the .from_formdata() caller in your request handler. With this PR, JSON decode errors will return empty form data instead of throwing an error. This is how the StartletteRequest .form() method handles malformed form bodies anyway so I don't think it will cause any backward compatibility issues. Let me know if this fix would address your problem.