[FIX] onboarding: fix panel's 'no' background

A traceback was shown when 'None' background was chosen to configure the
onboarding panel's background image (css class).

Supposing no one built anything with a `o_onboarding_False` class, we also
clean the result for a `False` value while we are here.

Task-3104723

closes odoo/odoo#108801

X-original-commit: 8a8da706e55073c48aa3cdc21201b4552230de87
Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>

addons/onboarding/models/onboarding_onboarding.py

@@ -94,12 +94,12 @@ class Onboarding(models.Model):
         self.ensure_one()
         values = {
             'bg_image': f'/web/image/onboarding.onboarding/{self.id}/panel_background_image',
+            'classes': f'o_onboarding_{self.panel_background_color}'
+                       if self.panel_background_color not in {False, 'none'} else '',
             'close_method': self.panel_close_action_name,
             'close_model': 'onboarding.onboarding',
             'steps': self.step_ids,
             'state': self.current_progress_id._get_and_update_onboarding_state(),
         }
-        if self.panel_background_color != 'none':
-            values.update(classes=f'o_onboarding_{self.panel_background_color}')
 
         return values