Remove possible null from stripos.

[tofumatt]

Summary

Addresses issue:

• PHP 8.1 Deprecation notice: strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated #7147

Relevant technical choices

I didn't quite catch in the original IB when reviewing this that $this->context->input()->filter( INPUT_GET, 'rest_route' ) might be null, which is what I'm thinking caused this issue: #7147 (comment)

The IB mentioned to make sure we didn't pass null, but I interpretted that literally and then merged the PR, as in testing it worked fine for me—I couldn't get the error described in the issue to appear.

Anyhow, this should fix it.

Note for merge reviewers: this was already in the release so targets main. After merging, make sure to merge main back into develop so this fix is in develop 🙂

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 5.6.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.

Merge Reviewer Checklist

• Ensure the PR has the correct target branch.
• Double-check that the PR is okay to be merged.
• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.

[github-actions]

Build files for 816a3d4 have been deleted.

[techanvil]

LGTM!