Improve robustness of getting router_subtype key

Signed-off-by: Xin Wang <xiwang5@microsoft.com>

Author: wangxin

tests/common/devices/multi_asic.py

@@ -83,7 +83,7 @@ def critical_services_tracking_list(self):
 
         # NOTE: Add mux to critical services for dualtor
         if (
-            self.facts['router_subtype'] == 'DualToR' and
+            self.facts.get('router_subtype', '') == 'DualToR' and
             self.facts.get('features', {}).get('mux', {}).get('state', '') == 'enabled'
         ):
             service_list.append("mux")