missing config option should error out with user prompt

[trammell]

In the current releases of 3.8.x, when running rasa data validate if config.yml does not exist, failing to specify the path to the config file with --config results in a stack trace:

Traceback (most recent call last):
  File "/.../python-3.10/bin/rasa", line 8, in <module>
    sys.exit(main())
  File "/.../python-3.10/lib/python3.10/site-packages/rasa/__main__.py", line 133, in main
2024-04-05 12:40:25 DEBUG    urllib3.connectionpool  - Starting new HTTPS connection (1): o251570.ingest.sentry.io:443
    cmdline_arguments.func(cmdline_arguments)
  File "/.../python-3.10/lib/python3.10/site-packages/rasa/cli/data.py", line 124, in <lambda>
    args.fail_on_warnings, args.max_history, _build_training_data_importer(args)
  File "/.../python-3.10/lib/python3.10/site-packages/rasa/cli/data.py", line 175, in _build_training_data_importer
    return TrainingDataImporter.load_from_config(
  File "/.../python-3.10/lib/python3.10/site-packages/rasa/shared/importers/importer.py", line 119, in load_from_config
    config = read_config_file(config_path)
  File "/.../python-3.10/lib/python3.10/site-packages/rasa/shared/utils/yaml.py", line 400, in read_config_file
    return read_validated_yaml(filename, CONFIG_SCHEMA_FILE, reader_type)
  File "/.../python-3.10/lib/python3.10/site-packages/rasa/shared/utils/yaml.py", line 530, in read_validated_yaml
    content = read_file(filename)
  File "/.../python-3.10/lib/python3.10/site-packages/rasa/shared/utils/io.py", line 113, in read_file
    with open(filename, encoding=encoding) as f:
TypeError: expected str, bytes or os.PathLike object, not NoneType

Proposed changes:

• change the value of none_is_valid to False in the call to get_validated_path()

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)