api: make build response inputs never return null
api: make build response inputs never return null

The SDKs are written to expect that an array is returned, so this
ensures that even when no inputs are provided to the build call, an
array is returned.

Closes chain/chainprv#713.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@jeffomatic

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain