Use an empty string as the default `TestRunConfig.log_path` (#689)

to avoid potential errors caused by `os.path.join`
diff --git a/mobly/config_parser.py b/mobly/config_parser.py
index 145e2d1..3372d7c 100644
--- a/mobly/config_parser.py
+++ b/mobly/config_parser.py
@@ -178,7 +178,8 @@
     """
 
     def __init__(self):
-        self.log_path = None
+        # Init value is an empty string to avoid string joining errors.
+        self.log_path = ''
         # Deprecated, use 'testbed_name'
         self.test_bed_name = None
         self.testbed_name = None