Don't persist $manualInputPrice when already existing

[thirdknown]

Describe the bug

When $manualInputPrice is already existing (loaded from ProductManualInputPriceRepository and not null), it is currently being persisted in the refresh() method of the ProductManualInputPriceFacade class. This may lead to unnecessary persistence and database updates.

Expected behavior

The $manualInputPrice should not be persisted when it is already existing, and only new instances of $manualInputPrice should be persisted.

Steps to reproduce

The issue can be found in the following code snippet from the ProductManualInputPriceFacade class:

https://github.com/shopsys/shopsys/blob/master/packages/framework/src/Model/Product/Pricing/ProductManualInputPriceFacade.php#L47-L58

Possible solution

Only persist the $manualInputPrice when it is newly created, i.e., when it is not loaded from the repository. Update the refresh() method to persist $manualInputPrice only when it is null.