[backport#15932] rpc: Use IsValidNumArgs in getblock

Summary:
https://github.com/bitcoin/bitcoin/pull/15932/commits/fa1c3591add184ecb43eb3b149f4833f241e3858

Partial backport of Core PR15932

Test Plan:

ninja check check-functional

run bitcoin-cli getblock and check that layout is nice and lined up

Reviewers: #bitcoin_abc, nakihito, deadalnix

Reviewed By: #bitcoin_abc, nakihito, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6296