Makes the command optional, since there are valid bitcoin-cli calls that have no command: - bitcoin-cli -? - bitcoin-cli -getinfo - ...
Partial backport of core PR12089 (commit fae7b14)
https://github.com/bitcoin/bitcoin/pull/12089/commits/fae7b14a0468fba43ca8ce87249be69e13cea608
Depends on D3458