Fix reboot without message (#250)

What I did
Fix gNOI cold reboot no message issue: sonic-net/sonic-buildimage#22545

How I did it
Use get instead of [] to cover no reboot message case

How to verify it
Tested gNOI warm reboot on a hardware switch.

root@sonic:~# docker exec -it gnmi bash
root@sonic:/# gnoi_client -notls -insecure -module System -rpc Reboot -jsonin '{"method":1}'
System Reboot
root@sonic:/# exit
root@sonic:~# journalctl -u sonic-hostservice.service -f
...
May 06 07:37:47 sonic python3[699742]: reboot: issue_reboot rpc called
May 06 07:37:47 sonic python3[699742]: reboot: Issuing cold reboot
...
May 06 07:38:18 sonic CCmisApi[700542]: reboot: Initiate shutdown command
client_loop: send disconnect: Broken pipe
Previous command output (if the output of a command-line utility has changed)
root@sonic:~# journalctl -u sonic-hostservice.service
...
May 06 01:28:14 sonic python3[514659]: reboot: issue_reboot rpc called
no more logs after that

New command output (if the output of a command-line utility has changed)
root@sonic:~# journalctl -u sonic-hostservice.service
...
May 06 07:37:47 sonic python3[699742]: reboot: issue_reboot rpc called
May 06 07:37:47 sonic python3[699742]: reboot: Issuing cold reboot
...
May 06 07:38:18 sonic CCmisApi[700542]: reboot: Initiate shutdown command
reboot is executed successfully

Author: jkmar

host_modules/reboot.py

@@ -185,7 +185,7 @@ def issue_reboot(self, options):
             return err, errstr
 
         # Sets reboot_status_flag to be in active state
-        self.populate_reboot_status_flag(True, int(time.time()), reboot_request["message"])
+        self.populate_reboot_status_flag(True, int(time.time()), reboot_request.get("message", ""))
 
         # Issue reboot in a new thread and reset the reboot_status_flag if the reboot fails
         try:

tests/host_modules/reboot_test.py

@@ -1,13 +1,11 @@
 """Tests for reboot."""
 
 import imp
+import json
 import sys
 import os
 import pytest
-import datetime
 import logging
-import time
-from itertools import repeat
 
 if sys.version_info >= (3, 3):
     from unittest import mock
@@ -275,6 +273,25 @@ def test_issue_reboot_success_cold_boot(self):
             )
             mock_thread.return_value.start.assert_called_once_with()
 
+    def test_issue_reboot_success_cold_no_message(self):
+        with (
+            mock.patch("threading.Thread") as mock_thread,
+            mock.patch("reboot.Reboot.validate_reboot_request", return_value=(0, "")),
+        ):
+            request = json.loads(VALID_REBOOT_REQUEST_COLD)
+            del request["message"]
+            request_str = json.dumps(request)
+
+            self.reboot_module.populate_reboot_status_flag()
+            result = self.reboot_module.issue_reboot([request_str])
+            assert result[0] == 0
+            assert result[1] == "Successfully issued reboot"
+            mock_thread.assert_called_once_with(
+                target=self.reboot_module.execute_reboot,
+                args=(REBOOT_METHOD_COLD_BOOT_ENUM,),
+            )
+            mock_thread.return_value.start.assert_called_once_with()
+
     def test_issue_reboot_success_halt(self):
         with (
             mock.patch("threading.Thread") as mock_thread,