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

chrome/app/policy/policy_templates.json

 {
 # policy_templates.json - Metafile for policy templates
 #
 # The content of this file is evaluated as a Python expression.
 #
 # This file is used as input to generate the following policy templates:
 # ADM, ADMX+ADML, MCX/plist and html documentation.
 #
 # Policy templates are user interface definitions or documents about the
 # policies that can be used to configure Chrome. Each policy is a name-value
@@ skipping 3752 matching lines @@
       'type': 'string',
       'schema': { 'type': 'string' },
       'supported_on': ['chrome_os:22-'],
       'device_only': True,
       'features': {
         'dynamic_refresh': True,
       },
       'example_value': 'America/Los_Angeles',
       'id': 158,
       'caption': '''Timezone''',
-      'desc': '''Specifies the timezone to be used for the device. Users can override the specified timezone for the current session. However, on logout it is set back to the specified timezone. If an invalid value is provided, the policy is still activated using "GMT" instead.
+      'desc': '''Specifies the timezone to be used for the device. Users can override the specified timezone for the current session. However, on logout it is set back to the specified timezone. If an invalid value is provided, the policy is still activated using "GMT" instead. If an empty string is provided, the policy is ignored.
 
       If this policy is not used, 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.
 
       New devices start out with the timezone set to "US/Pacific".
 
       The format of the value follows the names of timezones in the "IANA Time Zone Database" (see "http://en.wikipedia.org/wiki/List_of_tz_database_time"). In particular, most timezones can be referred to by "continent/large_city" or "ocean/large_city".''',
     },
     {
       'name': 'ShowLogoutButtonInTray',
       'type': 'main',
@@ skipping 611 matching lines @@
       'desc': '''Text appended in parentheses to the policy name to indicate that it has been deprecated''',
       'text': 'deprecated',
     },
     'doc_recommended': {
       'desc': '''Text appended in parentheses next to the policies top-level container to indicate that those policies are of the Recommended level''',
       'text': 'Default Settings (users can override)',
     },
   },
   'placeholders': [],
 }