[FIX] website_form: set meta field

backport of 682ae1cc64

Before this commit, if you enable website_form_enable_metadata, that
will crash with a "KeyError: 'meta'"

This commit closes #24888

addons/website_form/controllers/main.py

@@ -107,6 +107,7 @@ class WebsiteForm(http.Controller):
             'record': {},        # Values to create record
             'attachments': [],  # Attached files
             'custom': '',        # Custom fields values
+            'meta': '',         # Add metadata if enabled
         }
 
         authorized_fields = model.sudo()._get_form_writable_fields()