Fix reboot without message

[jkmar]

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

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jkmar]

Please review @kwangsuk @anand-kumar-subramanian @mbalachandar @amrutasali @sachinholla @sneelam20