* Fix for special case where purchase packing list is root.

* Use getSourceAssetPriceCurrency
* Do not expand if no SourceAssetPrice

We need to be able to see the future without auto planned orders, else how are we going to update them?

erp5_mrp: Copy price_currency to Manufacturing Execution and Production Packing List (PSA specific?).

In PSA for ME: Problem Report is created from a ME which then build a Returned
IPL from scratch copying properties from the ME. After this Returned IPL is
root-expanded, Real Time Inventory Accounting Rule is expanded too and requires
price_currency.