[IMP] web: FieldSelection: no empty value if required

From this rev., the empty value isn't available in the options of
required selection fields, thus preventing the user to unset it.

Related to task #49262
Closes #24404

addons/web/static/src/js/fields/relational_fields.js

@@ -2166,6 +2166,19 @@ var FieldSelection = AbstractField.extend({
     isSet: function () {
         return this.value !== false;
     },
+    /**
+     * Listen to modifiers updates to hide/show the falsy value in the dropdown
+     * according to the required modifier.
+     *
+     * @override
+     */
+    updateModifiersValue: function () {
+        this._super.apply(this, arguments);
+        if (!this.attrs.modifiersValue.invisible && this.mode !== 'readonly') {
+            this._setValues();
+            this._renderEdit();
+        }
+    },
 
     //--------------------------------------------------------------------------
     // Private
@@ -2219,7 +2232,9 @@ var FieldSelection = AbstractField.extend({
                 return v[0] === false && v[1] === '';
             });
         }
-        this.values = [[false, this.attrs.placeholder || '']].concat(this.values);
+        if (!this.attrs.modifiersValue || !this.attrs.modifiersValue.required) {
+            this.values = [[false, this.attrs.placeholder || '']].concat(this.values);
+        }
     },
 
     //--------------------------------------------------------------------------

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

@@ -11558,6 +11558,41 @@ QUnit.module('relational_fields', {
         form.destroy();
     });
 
+    QUnit.test('required selection widget should not have blank option', function (assert) {
+        assert.expect(3);
+
+        this.data.partner.fields.feedback_value = {
+            type: "selection",
+            required: true,
+            selection : [['good', 'Good'], ['bad', 'Bad']],
+            default: 'good',
+            string: 'Good'
+        };
+        var form = createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch: '<form string="Partners">' +
+                        '<field name="feedback_value"/>' +
+                        '<field name="color" attrs="{\'required\': [(\'feedback_value\', \'=\', \'bad\')]}"/>' +
+                '</form>',
+            res_id: 1
+        });
+
+        form.$buttons.find('.o_form_button_edit').click();
+        assert.strictEqual(form.$('.o_field_widget[name=color]')[0].options.length, 3,
+            "non required selection field must have 3 options 1 blank option and 2 value options");
+        assert.strictEqual(form.$('.o_field_widget[name=feedback_value]')[0].options.length, 2,
+            "should have only 2 options without blank option");
+
+        // change value to update widget modifier values
+        form.$('.o_field_widget[name=feedback_value]').val('"bad"').trigger('change');
+        assert.strictEqual(form.$('.o_field_widget[name=color]')[0].options.length, 2,
+            "should have only 2 options");
+
+        form.destroy();
+    });
+
     QUnit.module('FieldMany2ManyTags');
 
     QUnit.test('fieldmany2many tags with and without color', function (assert) {