Fix issue: ERR healthd: Get unit status determine-reboot-cause-'LoadS…

…tate' (sonic-net#13697)

- Why I did it
Fix issue: ERR healthd: Get unit status determine-reboot-cause-'LoadState'. The error log is only seen on shutdown flow such as fast-reboot/warm-reboot.

In shutdown flow, 'LoadState' might not be available in systemctl status output, using [] might cause a KeyError.

- How I did it
Use dict.get instead of []

- How to verify it
Manual test

src/system-health/health_checker/sysmonitor.py

@@ -273,7 +273,7 @@ def get_unit_status(self, event):
 
             sysctl_show = self.run_systemctl_show(event)
 
-            load_state = sysctl_show['LoadState']
+            load_state = sysctl_show.get('LoadState')
             if load_state == "loaded":
                 status = sysctl_show['UnitFileState']
                 fail_reason = sysctl_show['Result']