Consider as stringable domain with empty clause

Closes #13414
(grafted from 49a55291512a957e496455f6474a9776e8c2c31c)

--HG--
branch : 6.0

sao/src/common.js

@@ -1132,7 +1132,7 @@
         },
         stringable: function(domain) {
             var stringable_ = function(clause) {
-                if (!clause) {
+                if (!clause || jQuery.isEmptyObject(clause)) {
                     return true;
                 }
                 var is_array = function(e) {

sao/tests/sao.js

@@ -2106,6 +2106,8 @@
         });
         var valid = ['name', '=', 'Doe'];
         var invalid = ['surname', '=', 'John'];
+        QUnit.ok(parser.stringable([]));
+        QUnit.ok(parser.stringable([[]]));
         QUnit.ok(parser.stringable([valid]));
         QUnit.ok(!parser.stringable([invalid]));
         QUnit.ok(parser.stringable(['AND', valid]));

tryton/tryton/tests/test_common_domain_parser.py

@@ -593,6 +593,8 @@ class DomainParserTestCase(TestCase):
                 })
         valid = ('name', '=', 'Doe')
         invalid = ('surname', '=', 'John')
+        self.assertTrue(dom.stringable([]))
+        self.assertTrue(dom.stringable([[]]))
         self.assertTrue(dom.stringable([valid]))
         self.assertFalse(dom.stringable([invalid]))
         self.assertTrue(dom.stringable(['AND', valid]))