Fix a bug in idempotent check. (sonic-net#755)

Currently it misses the case where data for a new key is empty.

scripts/configlet

@@ -101,11 +101,16 @@ def init():
 def db_update(t, k, lst):
     init()
     to_upd = False
-    data = db.get_entry(t, k)
-    for i in lst.keys():
-        if not data.has_key(i) or data[i] != lst[i]:
-            to_upd = True
-            break
+    ct_keys = db.get_keys(t)
+    tuple_k = db.deserialize_key(k[0])
+    if tuple_k in ct_keys:
+        data = db.get_entry(t, k)
+        for i in lst.keys():
+            if not data.has_key(i) or data[i] != lst[i]:
+                to_upd = True
+                break
+    else:
+        to_upd = True
 
     if to_upd:
         db.mod_entry(t, k, lst)