nbconvert for python 3.9 crashing during the templateexporter.py

[ale-franc]

While trying to convert a minimum jupyter notebook we got the following message.

$ /opt/python/3.9.14/bin/python3.9 -m nbconvert --execute --stdout  --to=html /net/.../fs0/home/ias4ca/Untitled3.ipynb
Traceback (most recent call last):
  File "/opt/python/3.9.14/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/opt/python/3.9.14/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/ias4ca/.local/lib/python3.9/site-packages/nbconvert/__main__.py", line 5, in <module>
    main()
  File "/opt/python/3.9.14/lib/python3.9/site-packages/jupyter_core/application.py", line 283, in launch_instance
    super().launch_instance(argv=argv, **kwargs)
  File "/opt/python/3.9.14/lib/python3.9/site-packages/traitlets/config/application.py", line 1075, in launch_instance
    app.start()
  File "/home/ias4ca/.local/lib/python3.9/site-packages/nbconvert/nbconvertapp.py", line 420, in start
    self.convert_notebooks()
  File "/home/ias4ca/.local/lib/python3.9/site-packages/nbconvert/nbconvertapp.py", line 585, in convert_notebooks
    cls = get_exporter(self.export_format)
  File "/home/ias4ca/.local/lib/python3.9/site-packages/nbconvert/exporters/base.py", line 107, in get_exporter
    if getattr(exporter(config=config), "enabled", True):
  File "/home/ias4ca/.local/lib/python3.9/site-packages/nbconvert/exporters/templateexporter.py", line 350, in __init__
    super().__init__(config=config, **kw)
  File "/home/ias4ca/.local/lib/python3.9/site-packages/nbconvert/exporters/exporter.py", line 123, in __init__
    self._init_preprocessors()
  File "/home/ias4ca/.local/lib/python3.9/site-packages/nbconvert/exporters/templateexporter.py", line 535, in _init_preprocessors
    conf = self._get_conf()
  File "/home/ias4ca/.local/lib/python3.9/site-packages/nbconvert/exporters/templateexporter.py", line 557, in _get_conf
    conf = recursive_update(conf, json.load(f))
  File "/opt/python/3.9.14/lib/python3.9/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/opt/python/3.9.14/lib/python3.9/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/opt/python/3.9.14/lib/python3.9/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/opt/python/3.9.14/lib/python3.9/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

$ jupyter --version
Selected Jupyter core packages...
IPython          : 8.18.1
ipykernel        : 6.29.3
ipywidgets       : 8.1.2
jupyter_client   : 7.4.9
jupyter_core     : 5.7.1
jupyter_server   : 2.13.0
jupyterlab       : 3.6.7
nbclient         : 0.9.0
nbconvert        : 7.16.4
nbformat         : 5.9.2
notebook         : 6.5.6
qtconsole        : 5.5.1
traitlets        : 5.14.1

cc: @rstub