Entity price getters need to account for empty field data

We have price getters on the product (getPrice), line item (getUnitPrice, getTotalPrice), order (getTotalPrice), payment (getAmount, getRefundedAmount).
Each of these always returns a Price object. But the underlying field items can be empty, since Drupal entities don't need to be in a valid state when initialized or saved. If the field item is empty, then the getter should return NULL, not a Price object (because the Price object can't be valid, and because that's what other similar getters do).