Missing handling for dummy output notes

[murisi]

The namadac client sometimes creates dummy output notes/output descriptions in order to disassociate the number of output descriptions in a Transaction from the actual number shielded outputs. These output descriptions do not correspond to any output descriptor information objects in the builder, their value is 0, and their asset type pre-encoding is dummy. The cryptographic checks should support the presence of these.

See attached a test vector containing a dummy note: dummy_note_testvector.txt . Note that there is are two output descriptions even though there is only one output description information object. Also note that most of the randomness parameters are not used. See also the code that creates them here: https://github.com/anoma/masp/blob/8d83b172698098fba393006016072bc201ed9ab7/masp_primitives/src/transaction/components/sapling/builder.rs#L1055-L1090 for how namadac constructs these output descriptions.

[ainhoa-a]

Can be closed once #69 gets merged