Check if the issue is closed.

    issue_key
        The JIRA iD of the ticket to close.

    Returns:

    - ``True``: the ticket exists and it is closed.
    - ``False``: the ticket exists and it has not been closed.
    - ``None``: the ticket does not exist.

    CLI Example:

    .. code-block:: bash

        salt '*' jira.issue_closed NE-123

def issue_closed(issue_key,
                 server=None,
                 username=None,
                 password=None):
    '''
    Check if the issue is closed.

    issue_key
        The JIRA iD of the ticket to close.

    Returns:

    - ``True``: the ticket exists and it is closed.
    - ``False``: the ticket exists and it has not been closed.
    - ``None``: the ticket does not exist.

    CLI Example:

    .. code-block:: bash

        salt '*' jira.issue_closed NE-123
    '''
    if not issue_key:
        return None
    jira_ = _get_jira(server=server,
                      username=username,
                      password=password)
    try:
        ticket = jira_.issue(issue_key)
    except jira.exceptions.JIRAError:
        # Ticket not found
        return None
    return ticket.fields().status.name == 'Closed'