Fix handling of potential empty gitiles result.

This fixes an issue introduced by
crrev.com/b40a4514 where a 404 result
might return None contents.

R=agable@google.com, phajdan@google.com

Bug:774355
Change-Id: I993a029adb8d92cb27e043cd3d9b17cc8aee2432
Reviewed-on: https://chromium-review.googlesource.com/717937
Commit-Queue: Michael Moss <mmoss@chromium.org>
Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>

recipes/recipe_modules/gitiles/api.py

@@ -137,4 +137,6 @@ class Gitiles(recipe_api.RecipeApi):
         fmt='text',
         add_json_log=False,
         **kwargs)
+    if step_result.json.output['value'] is None:
+      return None
     return base64.b64decode(step_result.json.output['value'])

recipes/recipe_modules/gitiles/examples/full.py

@@ -4,6 +4,7 @@
 
 DEPS = [
     'gitiles',
+    'recipe_engine/json',
     'recipe_engine/properties',
 ]
 
@@ -62,6 +63,6 @@ def GenTests(api):
       )
       + api.step_data(
           'fetch master:NONEXISTENT',
-          api.gitiles.make_encoded_file('')
+          api.json.output({'value': None})
       )
   )