[FIX] web_editor: fix required html fields

Issue

    - Studio
    - Add an required html field
    - Create new record
    - Leave it empty

    It is saved

Cause

    The isSet function of FieldHtml widget, have a "/" in the <br> tag
    and summernote empty = <p><br></p>. If you add a space, the value
    becomes <p>&nbsp;&nbsp;   <p>.

Solution

    Remove the "/" and the spaces, check if value != <p></p> &&
    <p><br></p>

OPW-2183687

closes odoo/odoo#44427

X-original-commit: c984b1b1
Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>