test: fix bitcoind already running warnings on macOS
test: fix bitcoind already running warnings on macOS

On macOS, pidof installed via brew returns b'' rather than None.
Account for this, to remove spurious warnings from the test_runner.


Former-commit-id: 404e04f027539d0c603915b246c0ce344be424df
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin