Empty value in O2M should remove existing record

The changeset 5efdef4c2c7d wrongly skip the removal of existing record in the
group. So it was not possible to remove the last record of the field.

issue5878
review34271002

src/model.js

@@ -1819,10 +1819,11 @@
             var group = record._values[this.name];
             var prm = jQuery.when();
             if (jQuery.isEmptyObject(value)) {
-                return prm;
+                value = [];
             }
             var mode;
-            if (!isNaN(parseInt(value[0], 10))) {
+            if (jQuery.isEmptyObject(value) ||
+                    !isNaN(parseInt(value[0], 10))) {
                 mode = 'list ids';
             } else {
                 mode = 'list values';