Fix default value and check of last_timestamp of Sequence

The client integer widget does not allow to display float value. So the default
value of last_timestamp is replaced by None which fails later in the check.
Also the check must not validate when it is not filled because in Python3 None
and int are unorderable.

issue5591
review28571002

trytond/ir/sequence.py

@@ -150,7 +150,7 @@ class Sequence(ModelSQL, ModelView):
 
     @staticmethod
     def default_last_timestamp():
-        return 0.0
+        return 0
 
     @staticmethod
     def default_code():
@@ -248,7 +248,8 @@ class Sequence(ModelSQL, ModelView):
 
         for sequence in sequences:
             next_timestamp = cls._timestamp(sequence)
-            if sequence.last_timestamp > next_timestamp:
+            if (sequence.last_timestamp is not None
+                    and sequence.last_timestamp > next_timestamp):
                 cls.raise_user_error('future_last_timestamp', (
                         sequence.rec_name,))