OpenSearch fix for: passing null is deprecated

[Fedik]

Pull Request for Issue #38427 .

Summary of Changes

Fix PHP 8 deprecation warning: Passing null to parameter ($string) of type string is deprecated

Testing Instructions

Use php 8+, error_reporting maximum Apply patch. Open smart search page. Add ?format=opensearch to URL, and open. Inspect the result.

Actual result BEFORE applying this Pull Request

deprecation warning

Expected result AFTER applying this Pull Request

no deprecation warning

Documentation Changes Required

no

[brianteeman]

I have tested this item :white_check_mark: successfully on c3636c1a97b5d5d90f87571f730d0369c0fbd867

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[Quy]

I have tested this item :white_check_mark: successfully on c3636c1a97b5d5d90f87571f730d0369c0fbd867

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38431.}

[Quy]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38431.}

[wilsonge]

Thanks!