Fix ensure_directory to handle empty strings

When save_path is a filename without directory, os.dirname(save_path) is
the empty string and ensure_directory fails. This patch handles the
empty string by checking if the directory variable is set at all before
calling os.makedirs with it.

Author: allo-

acestep/pipeline_ace_step.py

@@ -82,7 +82,7 @@
 
 def ensure_directory_exists(directory):
     directory = str(directory)
-    if not os.path.exists(directory):
+    if directory and not os.path.exists(directory):
         os.makedirs(directory)