Allow None value for default value check in policy presubmit

Previously, the check failed if the default value was set to None
explicitly. This CL changes the check to fail only if the default value
is not set.

Change-Id: I2eb179f306e340b73e24a08f9bfe63bc07706b3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533288
Commit-Queue: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Chris Sharp <csharp@chromium.org>
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826742}

components/policy/tools/syntax_check_policy_template_json.py

@@ -1552,8 +1552,7 @@ class PolicyTemplateChecker(object):
 
       # TODO(crbug.com/1139046): This default check should apply to all
       # policies instead of just new ones.
-      if self._NeedsDefault(new_policy) and new_policy.get('default',
-                                                           None) == None:
+      if self._NeedsDefault(new_policy) and not 'default' in new_policy:
         self._Error("Definition of policy %s must include a 'default'"
                     " field." % (new_policy_name))