[FIX] web: prevents not dirty forms saving

Purpose
=======
Prevents auto save of not dirty forms with
text fields on page reload and tab/browser closing.

Specifications
==============
The text field default value is making Odoo consider
the form as dirty even when it's not which triggers
auto saving for not dirty forms.

Change the text field widget setDirty condition to
prevent considering empty textarea string dirty.
Typically, if the previous value was false, false and
empty string should be considered as the same value.

Task-3031570

closes odoo/odoo#106439

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>