qa: Make TestNodeCLI command optional in send_cli

Summary:

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

Test Plan:

./test/functional/test_runner.py interface_bitcoin_cli

wallet_multiwallet

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3459