UnboundLocalError for unknown output types

[kokes]

If a cell has an unexpected output_type, we get an UnboundLocalError, because a variable used is unassigned. The code in question is this block.

$ nbterm foo.ipynb
Traceback (most recent call last):
  File "/Users/okokes/Notebooks/.dev/bin/nbterm", line 8, in <module>
    sys.exit(cli())
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/nbterm/nbterm.py", line 43, in cli
    typer.run(main)
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/typer/main.py", line 859, in run
    app()
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/typer/main.py", line 214, in __call__
    return get_command(self)(*args, **kwargs)
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/typer/main.py", line 497, in wrapper
    return callback(**use_params)  # type: ignore
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/nbterm/nbterm.py", line 29, in main
    nb = Notebook(notebook_path, no_kernel=no_kernel or False, save_path=save_path)
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/nbterm/notebook.py", line 61, in __init__
    self.read_nb()
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/nbterm/format.py", line 15, in read_nb
    self.cells = [
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/nbterm/format.py", line 16, in <listcomp>
    Cell(self, cell_json=cell_json) for cell_json in self.json["cells"]
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/nbterm/cell.py", line 98, in __init__
    output_text, output_height = get_output_text_and_height(
  File "/Users/okokes/Notebooks/.dev/lib/python3.9/site-packages/nbterm/cell.py", line 45, in get_output_text_and_height
    text_list.append(text)
UnboundLocalError: local variable 'text' referenced before assignment

(output_type is display_data in my case)

I have a fix ready, I'll submit it shortly.

[davidbrochart]

Fixed in #20.