Fix the Keyerror issue caused by none conn_graph_fact in VS set-up (s…

…onic-net#12751)

What is the motivation for this PR?
To adapt to kvm testbed, we need to fix the issue -- conn_graph_fact is none in VS set-up.
In this PR, we use method get of a dict to get the value instead of [] to avoid key error when getting value from an empty dict conn_graph_fact.

Also, in this PR, after fixing this issue, we can add three platform tests into PR test.

.azure-pipelines/pr_test_scripts.yaml

@@ -352,6 +352,9 @@ onboarding_dualtor:
   - dualtor_mgmt/test_toggle_mux.py
 =======
   - console/test_console_availability.py
+  - platform_tests/test_power_off_reboot.py
+  - platform_tests/test_sequential_restart.py
+  - platform_tests/test_xcvr_info_in_db.py
 
 >>>>>>> Add a separate non-blocking PR check for onboarding new tests (#12729)