RPC: Show txinwitness as json array

[jl2012]

Currently null stack item is not shown in txinwitness, except as an extra space. This patch shows null stack item as "null"

An alternative would be using "|" instead of space as separator so it's easier to see null character

[sipa]

Alternatively, why not convert the witness script stack to a JSON array of hex-encoded strings?

[jl2012]

revised and tested

[sipa]

Squashed into segwit4.