ENH: Add drop_blank_outputs input field

Author: effigies

nipype/interfaces/io.py

@@ -1044,6 +1044,10 @@ class DataGrabberInputSpec(DynamicTraitedSpec, BaseInterfaceInputSpec):
         True,
         usedefault=True,
         desc='Generate exception if list is empty for a given field')
+    drop_blank_outputs = traits.Bool(
+        False, usedefault=True,
+        desc="Remove ``None`` entries from output lists"
+        )
     sort_filelist = traits.Bool(
         mandatory=True, desc='Sort the filelist that matches the template')
     template = Str(