[FIX] web_editor: entering a hax color triggers traceback

Before this commit:

In mass_mailing entering a hax color for text triggers traceback.

After this commit:

Now, the traceback will not be triggered and color will be applied to the text

Task-3271517

closes odoo/odoo#120507

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>