[backport#15930] wallet: Use IsValidNumArgs in getwalletinfo rpc

Summary:
https://github.com/bitcoin/bitcoin/pull/15930/commits/fad40ec9151248c6e8225e14980424f581d23e02

Depends on D6443

Partial backport of Core PR15930

Test Plan:

ninja
./src/bitcoind -regtest -daemon
./src/bitcoin-cli getwalletinfo help

check that it looks alright

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6444