Ignoring the timezone policy if it is set to an empty string.

chrome/browser/policy/proto/chrome_device_policy.proto

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package enterprise_management;
 
@@ skipping 193 matching lines @@
   // Specifies the URLs to be loaded on login to the anonymous account used if
   // the device is in RETAIL mode.
   repeated string start_up_urls = 1;
 }
 
 message SystemTimezoneProto {
   // Specifies an owner-determined timezone that applies to the login screen and
   // all users. Valid values are listed in "timezone_settings.cc". Additionally,
   // timezones from the "IANA Time Zone Database" (e.g. listed on wikipedia)
   // that are equivalent to one of the timezones in "timezone_settings.cc" are
-  // valid. In case of an invalid value (e.g. the empty string), the setting is
-  // still activated with a fallback timezone (currently "GMT").
-  // Only if no value is provided, the timezone device setting is inactive. In
-  // that case, the currently active timezone will remain in use however users
-  // can change the timezone and the change is persistent. Thus a change by one
-  // user affects the login-screen and all other users.
+  // valid. In case of an invalid value, the setting is still activated with a
+  // fallback timezone (currently "GMT"). In case of an empty string or if no
+  // value is provided, the timezone device setting is inactive. In that case,
+  // the currently active timezone will remain in use however users can change
+  // the timezone and the change is persistent. Thus a change by one user
+  // affects the login-screen and all other users.
   optional string timezone = 1;
 }
 
 // Describes a single device-local account.
 message DeviceLocalAccountInfoProto {
   // Identifier for the device-local account. Account identifiers loosely follow
   // the conventions for email addresses as specified by RFC 5322, e.g. local
   // (user name) and domain parts, separated by the '@' character.
   // Policy-configured device-local accounts are on the same domain that the
   // device is enrolled to. In contrast to RFC 5322 (which is more restrictive),
@@ skipping 32 matching lines @@
   optional AppPackProto app_pack = 14;
   optional ForcedLogoutTimeoutsProto forced_logout_timeouts = 15;
   optional ScreenSaverProto login_screen_saver = 16;
   optional AutoUpdateSettingsProto auto_update_settings = 17;
   optional StartUpUrlsProto start_up_urls = 18;
   optional PinnedAppsProto pinned_apps = 19;
   optional SystemTimezoneProto system_timezone = 20;
   optional DeviceLocalAccountsProto device_local_accounts = 21;
   optional AllowRedeemChromeOsRegistrationOffersProto allow_redeem_offers = 22;
 }