[FIX] web: missing required attribute for html field

To reproduce
============

* referrals -> configuration -> rewards
* try creating a new reward keeping description empty
* here description is required html field
* traceback on clicking save

Problem
=======
in case of html field, `this.data[fieldName]` is an object `Markup{''}`,
so `!this.data[fieldName]` is always `false` as objects are considered
as true value in JS.

Solution
========
in case of html, check the length of the Markup object

opw-3469539

closes odoo/odoo#134119

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>
Co-authored-by: Niyas Raphy <https://github.com/niyasraphy>