fix for UUID set on invalid object

[brian-davis]

Bug: When using :should_generate_new_friendly_id? to enable slug update functionality, on a model which has option friendly_id :title, use: :slugged, and an existing validates :title, presence: true, the UUID is being set as the slug while the object is in an invalid state. In a rails app, this will be set as the action attribute in a form_with form, causing errors.

Fix: Add an errors.key?(friendly_id_config.base) guard clause to :unset_slug_if_invalid callback method.