[php 8.1 compat] Avoid CRM_Utils_System::url null for $query param

[demeritcowboy]

Overview

Brings in line with #24176.

Before

Some inconsistency between null and '' on drupal 7, more so when not using clean urls.

After

More like before, but more php8.1 better.

Technical Details

Comments

It doesn't look like wordpress or joomla need a similar update since they don't use isset on $query.

[civibot]

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