Only add the child config_dir option if it is set in the parent.

qipipe/pipeline/workflow_base.py

@@ -181,6 +181,7 @@ def depict_workflow(self, workflow):
     def _child_options(self):
         """
         Collects the following options for creating a child workflow:
+
         * project
         * config_dir
         * base_dir
@@ -189,13 +190,16 @@ def _child_options(self):
 
         :return: the options sufficient to create a child workflow
         """
-        return dict(
+        opts = dict(
             project=self.project,
             base_dir=self.base_dir,
-            config_dir=self.config_dir,
             dry_run=self.dry_run,
             distributable=self.is_distributable
         )
+        if self.config_dir:
+            opts['config_dir'] = self.config_dir
+
+        return opts
 
     def _load_configuration(self, name):
         """