Haven 4.2 - conversion fixes

[Tay8NWWFKpz9JT4NXU0w]

there were several issues with the transaction construction and validation, which are fixed with this PR:

the wallet software was still trying to request XHV collateral with 0 amount, even though it was not needed
the output collection from pool 2 was not working properly when the block at the cut-off height did not have a particular asset outputs, like XUSD
in the validation, masks were required even for transfers
the "reverse" engineering of the net amount in color C used for conversion was not completely correct