Fix case where we don't pass in a test base.

bin/au_test_harness/cros_au_test_harness.py

Index: bin/au_test_harness/cros_au_test_harness.py
diff --git a/bin/au_test_harness/cros_au_test_harness.py b/bin/au_test_harness/cros_au_test_harness.py
index ca0d7b63ebf8f8dd642b037e7c7776e2814c2e0e..10c6abcd460af15797e1310fc2c081b4541420c5 100755
--- a/bin/au_test_harness/cros_au_test_harness.py
+++ b/bin/au_test_harness/cros_au_test_harness.py
@@ -236,7 +236,8 @@ def main():
   if options.clean: _CleanPreviousWork(options)
 
   # Make sure we have a log directory.
-  if not os.path.exists(options.test_results_root):
+  if options.test_results_root and not os.path.exists(
+      options.test_results_root):
     os.makedirs(options.test_results_root)
 
   # Pre-generate update modifies images by adding public keys to them.