from typing import Optional


def read_text_file(file_name: str) -> Optional[str]:
    """
    Reads file and returns its text
    :param file_name: name of a file to read
    :return: text of the file
    """

    if not file_name:
        return None

    # encoding='utf-8-sig' is used for omitting \ufeff symbol
    # in the beginning of the string after reading from file
    with open(file_name, 'r', encoding='utf-8-sig') as text_file:
        text = text_file.read()

    return text