Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

I encountered a fatal error when accessing the Coming Soon page on a site that is also running the Optimization Detective plugin. The reason is that Optimization Detective adds a template_include filter the same as WooCommerce does, but Optimization Detective adds string type hint for the callback function since the filter defines the $template param as a string. Nevertheless, ComingSoonRequestHandler::handle_template_include() seems to disregard this by returning string|null. The fix seems to simply return an empty string instead of null. I've also fixed this in Optimization Detective by removing the type hint in WordPress/performance#1565, but it could be a problem with other plugins as well.

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

1. Enable the Coming Soon page
2. Activate the Optimization Detective plugin
3. Attempt to access the Shop page while being logged-out

Changelog entry

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