Fix child_of domain on empty MPTT

When the MPTT search returns no row, the SQL query generated has an invalid
syntax because it uses an empty Or() clause. We replace this empty Or() by a
Literal(False) to select nothing.

issue5011
review18651002

trytond/model/fields/many2one.py

@@ -1,7 +1,7 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 from types import NoneType
-from sql import Query, Expression
+from sql import Query, Expression, Literal
 from sql.operators import Or
 
 from .field import Field, SQLType
@@ -109,6 +109,8 @@ class Many2One(Field):
         where = Or()
         for l, r in cursor.fetchall():
             where.append((left >= l) & (right <= r))
+        if not where:
+            where = Literal(False)
         if operator == 'not child_of':
             return ~where
         return where