CrOS: Do not set an empty policy hostname

This is in preparation for adding UI to allow users
to set a custom DHCP hostname. Currently the policy
code is always setting the hostname, even if
kDeviceHostnameTemplate is not set by policy,
which overrides any other custom set hostname.

Bug: 1103272
Change-Id: Ia99ee58c59ae1f38e8fe10ed693997dead861c4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2320991
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Omar Morsi <omorsi@google.com>
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799393}

chrome/browser/chromeos/policy/hostname_handler.cc

@@ -119,8 +119,12 @@ void HostnameHandler::OnDeviceHostnamePropertyChanged() {
 void HostnameHandler::
     OnDeviceHostnamePropertyChangedAndMachineStatisticsLoaded() {
   std::string hostname_template;
-  cros_settings_->GetString(chromeos::kDeviceHostnameTemplate,
-                            &hostname_template);
+  if (!cros_settings_->GetString(chromeos::kDeviceHostnameTemplate,
+                                 &hostname_template)) {
+    // Do not set an empty hostname (which would overwrite any custom hostname
+    // set) if DeviceHostnameTemplate is not specified by policy.
+    return;
+  }
 
   const std::string serial = chromeos::system::StatisticsProvider::GetInstance()
                                  ->GetEnterpriseMachineID();