Check that author variable is of the expected type

[donnapep]

Fixes an issue reported by Team51:

When using Safety Net on a site with the Sensei plugin, the following warning was encountered:

Warning: Attempt to read property "display_name" on bool in /plugins/sensei-lms/includes/class-sensei-modules.php on line 2653

Proposed Changes

Check that $author is a WP_User object before trying to access the display_name property.

Though I wasn't able to reproduce the reported issue, this seems like a safe change.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• Code is tested on the minimum supported PHP and WordPress versions