[FIX] web: prevent virtual datapoint creation when no default value

Previously, it used to create a virtual datapoint even though there was
no change on this field. If the field was a many2one, this datapoint
triggered a name_get call afterwards, but giving the virtual id of
the datapoint since there is no real record. The solution is to avoid
creating a virtual datapoint when it is not needed at all, thus fixing
the wrong name_get call.

Steps to reproduce the bug:
- Enable lots in Iventory
- Choose a product which is a component of another (through mrp bom)
- Set this product as tracked by lot
- Set this product inventory quantity to 0
- Create a new manufacturing order for the product for which it is a
  component
- Click on "Check Availability"
- Click on Produce
- You'll get an error as the web client tries to fetch the name_get of
  this datapoint even though the record does not exist.

addons/web/static/src/js/views/basic/basic_model.js

@@ -2877,6 +2877,9 @@ var BasicModel = AbstractModel.extend({
 
                                 r._changes = _.defaults(value[2], r.data);
                                 for (var name in r._changes) {
+                                    if (r._changes[name] === null) {
+                                        continue;
+                                    }
                                     var isFieldInView = name in r.fields;
                                     if (isFieldInView && r.fields[name].type === 'many2one') {
                                         var rec = self._makeDataPoint({

addons/web/static/tests/fields/relational_fields_tests.js

@@ -922,6 +922,42 @@ QUnit.module('relational_fields', {
         });
     });
 
+    QUnit.test('list in form: create with one2many with many2one', function (assert) {
+        assert.expect(1);
+
+        var form = createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch: '<form>' +
+                    '<sheet>' +
+                        '<field name="p">' +
+                            '<tree editable="bottom">' +
+                                '<field name="display_name"/>' +
+                                '<field name="trululu"/>' +
+                            '</tree>' +
+                        '</field>' +
+                    '</sheet>' +
+                '</form>',
+            mockRPC: function (route, args) {
+                if (args.method === 'default_get') {
+                    return $.when({p: [[0, 0, {display_name: 'new record'}]]});
+                } else if (args.method === 'name_get') {
+                    // This should not be called at all and thus is not accounted for
+                    // in the assert.expect. If this is called, you broke this test.
+                    assert.notOk(_.str.startsWith(args.args[0][0], 'virtual_'),
+                        "should not call name_get for the m2o inside o2m which has no value");
+                }
+                return this._super.apply(this, arguments);
+            },
+        });
+
+        assert.strictEqual($('td.o_data_cell:first').text(), 'new record',
+            "should have created the new record in the o2m with the correct name");
+
+        form.destroy();
+    });
+
     QUnit.test('autocompletion in a many2one, in form view with a domain', function (assert) {
         assert.expect(1);