json: Improve error handling and adding ability to limit log output.

recipe_modules/json/example.py

 # 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.
 
 DEPS = [
   'json',
   'path',
   'python',
   'raw_io',
   'step',
@@ skipping 60 matching lines @@
   assert step_result.json.output == example_dict
 
   # invalid data gets rendered
   step_result = api.step('invalid json', [
     'python', api.resource('cool_script.py'),
     '{"here is some total\ngarbage',
     api.json.output(),
   ])
   assert step_result.json.output is None
 
+  # empty json data is explicit
+  step_result = api.step('empty json', [
+    'python', api.resource('cool_script.py'),
+    '',
+    api.json.output(),
+  ])
+  assert step_result.json.output is None
+
+  # empty json data is explicit
+  step_result = api.step('None json', [
+    'python', api.resource('cool_script.py'),
+    api.json.output(),
+  ])
+  assert step_result.json.output is None
+
 
 def GenTests(api):
   yield (
     api.test('basic')
     + api.step_data('echo1', stdout=api.json.output([1, 2, 3]))
     + api.step_data(
       'foo',
       api.json.output([1, 2, 3], name='1') +
       api.json.output(['x', 'y', FULLWIDTH_Z], name='2'),
     )
     + api.step_data(
       'leaking json',
       api.json.output({'x': 1, 'y': 2}),
     )
     + api.step_data(
       'invalid json',
       api.json.invalid('{"here is some total\ngarbage'),
     )
+    + api.step_data(
+      'empty json',
+      api.json.invalid(''),
+    )
   )