[FIX] web: various fixes for date and datetime in client

The field_utils date method have been modified recently, and the idea
behind those changes is that the BasicModel keeps at all time a moment
instance as its internal representation of a date or datetime.

But the parse and format methods did not properly support unset dates,
so we have to slighlty change them to support the case where their values is
not set.

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

@@ -479,6 +479,22 @@ QUnit.module('basic_fields', {
 
     QUnit.module('FieldDate');
 
+    QUnit.test('date field is empty if no date is set', function (assert) {
+        assert.expect(2);
+
+        var form = createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch:'<form string="Partners"><field name="date"/></form>',
+            res_id: 4,
+        });
+        var $span = form.$('span.o_form_field');
+        assert.strictEqual($span.length, 1, "should have one span in the form view");
+        assert.strictEqual($span.text(), "", "and it should be empty");
+
+    });
+
     QUnit.test('date field in edit mode', function (assert) {
         assert.expect(7);