Missing prices in the final year

Not a bug per se, but a feature of the way prices are calculated. Summed up by this comment:

> As you noted, there's a particular issue with missing prices in the final year. In "normal" years, if no existing assets produce a particular commodity, then we use candidate assets from the following year to calculate prices. In this way, any commodity that might need a price for investment decisions in the following year (i.e. as it could be produced/consumed) will have a price (noting that investment decisions in that year will use prices from the _previous year_). When it comes to the final year, there is no "following year" that allows us to use this candidates approach. This is okay because we don't need these prices in the model, but I understand it could be confusing to have missing prices in the final year. If we do need prices for these missing commodities, it seems to me that any method we use to get them would require treating the final year differently to the previous years, which seems a bit wrong.

_Originally posted by @tsmbland in [#1130](https://github.com/EnergySystemsModellingLab/MUSE2/issues/1130#issuecomment-3992171235)_