defensively checking for a missing widget_id

[masteradhoc]

Fixes #2910

Changes Proposed in this Pull Request

• Added an empty() guard on $args['widget_id'] in WP_Job_Manager_Widget::get_cached_widget() and cache_widget() to prevent PHP 8+ undefined array key warnings.
• When widget_id is absent, get_cached_widget() returns false (cache miss) and cache_widget() returns early — both fall back gracefully to normal widget rendering without caching.

Testing Instructions

1. Ensure a PHP 8.0+ environment with error reporting set to E_ALL.
2. Add a WP Job Manager widget (e.g. Recent Jobs or Featured Jobs) to a sidebar via Appearance → Widgets.
3. Load a frontend page that displays the widget.
4. Confirm no undefined array key "widget_id" PHP warnings appear in the error log or on screen.
5. Confirm the widget renders correctly with caching intact under normal conditions (i.e. $args['widget_id'] is present as expected in a standard WordPress widget context).

Release Notes

• Fixed PHP 8+ undefined array key warning in the widget caching methods.

New or Updated Hooks and Templates

None.

Deprecated Code

None.

Screenshot / Video

N/A — no visual change.