Test if there is something to write before calling write

ModelStorage.write requires at least a tuple of records, values.

issue5456
review21921002

statement.py

@@ -431,7 +431,8 @@ class Statement(Workflow, ModelSQL, ModelView):
             to_write.append({
                     'move': move.id,
                     })
-        Line.write(*to_write)
+        if to_write:
+            Line.write(*to_write)
 
         move_lines = []
         for move, statement, lines in moves:

tests/scenario_account_statement.rst

@@ -430,3 +430,9 @@ Test number of lines validation::
     >>> second_line.account = receivable
     >>> second_line.party = customer
     >>> statement.click('validate_statement')
+
+Validate empty statement::
+
+    >>> statement = Statement(name='empty', journal=statement_journal)
+    >>> statement.end_balance = statement.start_balance
+    >>> statement.click('validate_statement')