Incorrect de-duplication on addItemToOrder of OrderLines with custom fields

[michaelbromley]

Describe the bug
When executing the addItemToOrder mutation, if that ProductVariant is already in the Order, it should be incremented rather than a new OrderLine being created. The logic for this is here:

vendure/packages/core/src/service/services/order.service.ts

Lines 322 to 334 in 04a49bf

	let orderLine = order.lines.find(line => {
	return (
	idsAreEqual(line.productVariant.id, productVariantId) &&
	JSON.stringify(line.customFields) === JSON.stringify(customFields)
	);
	});
	if (!orderLine) {
	const newLine = this.createOrderLineFromVariant(productVariant, customFields);
	orderLine = await this.connection.getRepository(ctx, OrderLine).save(newLine);
	order.lines.push(orderLine);
	await this.connection.getRepository(ctx, Order).save(order, { reload: false });
	}

However, if one or more custom fields is set on the OrderLine entity, and none are set in the addItemToOrder mutation, the orderLine var on line 322 is incorrectly undefined.

To Reproduce
Steps to reproduce the behavior:

1. Define a custom field on OrderLine and start server
2. invoke addItemToOrder with variant id 1, quantity 1, no custom fields.
3. repeat step 2
4. See that there are 2 OrderLines in the Order where there should only be 1.

Expected behavior
Correct de-deduplication of order lines when the custom fields are empty.

Environment (please complete the following information):

• @vendure/core version: 0.16.1
• Nodejs version: any
• Database (mysql/postgres etc): any