Pset output scalar decoding bug.

[sanket1729]

The pset spec says that global scalars PSBT_ELEMENTS_GLOBAL_SCALAR must have value None, but as implemented they have value 0100 (a single vector containing the value zero)

In particular,

elements/src/psbt.h

Line 1425 in 04cedf3

SerializeToVector(s, std::vector<unsigned char>());

should directly write 0 instead of SerializeToVector(s, std::vector())

[jgriffiths]

Also, UnserializeFromVector should return immediately if expected_size == 0.