Update /wp-includes/kses.php to fix deprecated warnings

[RobertP3469]

Update /wp-includes/kses.php to fix deprecated warnings:

-- Backtrace from warning 'preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated' at /wp-includes/kses.php 1805:

This was the line of code causing the error:
$content = preg_replace( '/[\x00-\x08\x0B\x0C\x0E-\x1F]/', '', $content );

around line 1805, updated code to: $content = preg_replace('/[\x00-\x08\x0B\x0C\x0E-\x1F]/', '', $content ?? '');
The ?? '' operator ensures that if $content is null, it is replaced with an empty string ('') before being processed by preg_replace().

Trac ticket: https://core.trac.wordpress.org/ticket/63108

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