def check_tiramisu_layers(tiramisu_model=None):
    """Checks and returns the tiramisu layers on the tiramisu model string.

    Parameters
    ----------
    tiramisu_model : str or None, optional (default : None)
        The tiramisu model to be used.

    Returns
    -------
    tiramisu_layers : str
        String with the amount of layers. Empty string if tiramisu_model is
        None.
    """
    if tiramisu_model is not None:
        tiramisu_layers = tiramisu_model[8:]
    else:
        tiramisu_layers = ''
    return tiramisu_layers