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

recipe_modules/raw_io/api.py

 # -*- encoding: utf-8 -*-
 # Copyright 2014 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 from recipe_engine import recipe_api
 from recipe_engine import util as recipe_util
 
 import os
 import shutil
@@ skipping 76 matching lines @@
       self._backing_file = '/path/to/tmp/' + self.suffix.lstrip('.')
     else:  # pragma: no cover
       output_fd, self._backing_file = tempfile.mkstemp(suffix=self.suffix)
       os.close(output_fd)
     return [self._backing_file]
 
   def result(self, presentation, test):
     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:
           return self.decode(f.read())
       finally:
         if not self.leak_to:
           os.unlink(self._backing_file)
         self._backing_file = None
 
   def decode(self, result):
@@ skipping 124 matching lines @@
     """Returns a directory Placeholder for use as a step argument.
 
     If 'leak_to' is None, the placeholder is backed by a temporary dir with
     a suffix 'suffix'. The dir is deleted when the step finishes.
 
     If 'leak_to' is not None, then it should be a Path and placeholder
     redirects IO to a dir at that path. Once step finishes, the dir is
     NOT deleted (i.e. it's 'leaking'). 'suffix' is ignored in that case.
     """
     return OutputDataDirPlaceholder(suffix, leak_to, name=name)