fix deprecated warning in dashboard setup

[rrennick]

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:

This PR converts the task actionUrl to a string before it's used as a parameter to string handling functions.

How to test the changes in this Pull Request:

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

1. Update to PHP 8.1
2. Load the WP dashboard in trunk
3. See the following warning in the debug log

[16-Jun-2023 17:06:47 UTC] PHP Deprecated:  substr(): Passing null to parameter #1 ($string) of type string is deprecated in /Users/ronrennick/Sites/wc/wp-content/woocommerce/plugins/woocommerce/includes/admin/class-wc-admin-dashboard-setup.php on line 97

4. Switch to this branch & refresh the page
5. No warning should be logged.

[github-actions]

Test Results Summary

Commit SHA: 83e92da

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	259	0	0	2	0	261	0m 52s
E2E Tests	198	0	0	10	0	208	15m 9s

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To view the full API test report, click here.
To view the full E2E test report, click here.
To view all test reports, visit the WooCommerce Test Reports Dashboard.

[github-actions]

Hi @jorgeatorres,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[jorgeatorres]

LGTM 👍 Thank you @rrennick!