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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _get_env_vars_value(filename):
 """ If the user provided a file containing values of environment variables, this method will read the file and return its value :param string filename: Path to file containing environment variable values :return dict: Value of environment variables, if provided. None otherwise :raises InvokeContextException: If the file was not found or not a valid JSON """

        if not filename:
            return None

        # Try to read the file and parse it as JSON
        try:

            with open(filename, 'r') as fp:
                return json.load(fp)

        except Exception as ex:
            raise InvokeContextException("Could not read environment variables overrides from file {}: {}".format(
                                         filename,
                                         str(ex)))