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.

test/functional/test_runner.py

@@ -364,9 +364,10 @@ def main():
 def run_tests(*, test_list, src_dir, build_dir, tmpdir, jobs=1, enable_coverage=False, args=None, combined_logs_len=0, failfast=False, runs_ci, use_term_control):
     args = args or []
 
-    # Warn if bitcoind is already running (unix only)
+    # Warn if bitcoind is already running
+    # pidof might fail or return an empty string if bitcoind is not running
     try:
-        if subprocess.check_output(["pidof", "bitcoind"]) is not None:
+        if subprocess.check_output(["pidof", "bitcoind"]) not in [b'']:
             print("%sWARNING!%s There is already a bitcoind process running on this system. Tests may fail unexpectedly due to resource contention!" % (BOLD[1], BOLD[0]))
     except (OSError, subprocess.SubprocessError):
         pass