[FIX] web: empty field char should be set to false

With the new views, empty char fields were sent to the server as ''
(empty string) instead of false.  This is not the correct behaviour, and
actually cause some problems.  For example, if you have a char field
with a unicity constraint, then you cannot set it to '', since there is
only one record that can take that value.

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

@@ -2499,6 +2499,8 @@ var BasicModel = AbstractModel.extend({
                 changes[fieldName] = value ?
                     this.localData[value].model + ',' + this.localData[value].res_id :
                     false;
+            } else if (type === 'char' && changes[fieldName] === '') {
+                changes[fieldName] = false;
             } else if (changes[fieldName] === null) {
                 changes[fieldName] = false;
             }

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

@@ -718,6 +718,38 @@ QUnit.module('basic_fields', {
         form.destroy();
     });
 
+    QUnit.test('setting a char field to empty string is saved as a false value', function (assert) {
+        assert.expect(1);
+
+        var form = createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch: '<form string="Partners">' +
+                    '<sheet>' +
+                        '<group>' +
+                            '<field name="foo"/>' +
+                        '</group>' +
+                    '</sheet>' +
+                '</form>',
+            res_id: 1,
+            viewOptions: {mode: 'edit'},
+            mockRPC: function (route, args) {
+                if (args.method === 'write') {
+                    assert.strictEqual(args.args[1].foo, false,
+                        'the foo value should be false');
+                }
+                return this._super.apply(this, arguments);
+            }
+        });
+
+        form.$('input[type="text"].o_field_widget').val('').trigger('input');
+
+        // save
+        form.$buttons.find('.o_form_button_save').click();
+        form.destroy();
+    });
+
     QUnit.test('char field with size attribute', function (assert) {
         assert.expect(1);