Returns the payment logo to display based on the payment method

@param {Object} purchase - the purchase with which we are concerned
@return {string|null} the payment logo type, or null if no payment type is set.
function paymentLogoType( purchase ) {
	if ( isPaidWithCreditCard( purchase ) ) {
		return purchase.payment.creditCard.type;
	}

	if ( isPaidWithPayPalDirect( purchase ) ) {
		return 'placeholder';
	}

	return purchase.payment.type || null;
}