from typing import Optional


def validate_invest_per_capacity(invest_per_capacity: Optional[float]) -> Optional[float]:
    """
    Validates the invest per capacity of an object.
    Invest per capacity is always optional.

    :param invest_per_capacity: The invest per capacity of the object.
    :return: The validated invest per capacity.
    """
    if invest_per_capacity is None:
        # Skip validation if no value provided
        return None

    if invest_per_capacity < 0:
        raise ValueError("Invest per capacity must be zero or positive.")

    return invest_per_capacity