[json] improve output to get leak_to and better logging.

recipe_modules/raw_io/api.py

Index: recipe_modules/raw_io/api.py
diff --git a/recipe_modules/raw_io/api.py b/recipe_modules/raw_io/api.py
index ce3d5f587c12b38ccf7e6b828cfb0d32883bbca3..bf755399326de05a677a01fe05db62548d0a427c 100644
--- a/recipe_modules/raw_io/api.py
+++ b/recipe_modules/raw_io/api.py
@@ -94,7 +94,7 @@ class OutputDataPlaceholder(recipe_util.OutputPlaceholder):
     assert self._backing_file
     if test.enabled:
       self._backing_file = None
-      return self.decode(test.data)
+      return self.decode(test.data or '')
     else:  # pragma: no cover
       try:
         with open(self._backing_file, 'rb') as f: