Make step.log_filename None by default

This is the behavior we expect when checking to see if the step has a log filename later on, such as in `SphericalBaseStep`.

polaris/step.py

@@ -236,7 +236,7 @@ def __init__(self, test_case, name, subdir=None, cpus_per_task=1,
 
         # these will be set before running the step, dummy placeholders for now
         self.logger = logging.getLogger('dummy')
-        self.log_filename = ""
+        self.log_filename = None
 
         # output caching
         self.cached = cached