Fix an issue in policy presubmit script

When policy changing name, _CheckDeprecatedFutureField() function crash
as there is no |new_policy| instance.

Add a check in the first call to resolve the issue. Note that, for
renamed or new added policy, it should be verified by the second call
which always has the |new_policy| instance.

Bug: 1076560
Tbr: pastarmovj@chromium.org

Change-Id: I32d6e9b8b8b00cb9db4a1d34e0023aa8060fe6ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2284611
Commit-Queue: Owen Min <zmin@chromium.org>
Reviewed-by: Saurabh Nijhara <snijhara@google.com>
Reviewed-by: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785756}

components/policy/tools/syntax_check_policy_template_json.py

@@ -1357,8 +1357,9 @@ class PolicyTemplateChecker(object):
             MergeDict(new_released_platforms, new_rolling_out_platform),
             current_version, original_policy['name'])
 
-      self._CheckDeprecatedFutureField(original_policy, new_policy,
-                                       original_policy['name'])
+      if new_policy:
+        self._CheckDeprecatedFutureField(original_policy, new_policy,
+                                         original_policy['name'])
 
     # Check brand new policies:
     for new_policy_name in set(