[pytest]: Check if appl DB exists before deleting (sonic-net#1983)

*Add a new helper method for deleting appl DBs, and use it anytime appl DB needs to be deleted
Signed-off-by: Lawrence Lee <lawlee@microsoft.com>

tests/conftest.py

@@ -410,10 +410,15 @@ def reset_dbs(self):
         self.flex_db = None
         self.state_db = None
 
-    def destroy(self) -> None:
+    def del_appl_db(self):
+        # APPL DB may not always exist, so use this helper method to check before deleting
         if getattr(self, 'appldb', False):
             del self.appldb
 
+
+    def destroy(self) -> None:
+        self.del_appl_db()
+
         # In case persistent dvs was used removed all the extra server link
         # that were created
         if self.persistent:
@@ -583,8 +588,7 @@ def ctn_restart(self) -> None:
         self.ctn.restart()
 
     def restart(self) -> None:
-        if self.appldb:
-            del self.appldb
+        self.del_appl_db()
 
         self.ctn_restart()
         self.check_ready_status_and_init_db()