PHP 8.1 compatibility - deprecated warnings

[scr4bble]

There are multiple parts of the library that started producing warnings after upgrading PHP to version 8.1
One of these warnings is:

PHP Deprecated:  htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated in /paxflow/templatesc/__Mustache_365e16969d0bf799e38423c6770dd045.php on line 693

Generated by:

   691	                $buffer .= $indent . '				<td>';
   692	                $value = $this->resolveValue($context->find('mail'), $context);
   693	                $buffer .= htmlspecialchars($value, 2, 'UTF-8');
   694	                $buffer .= '</td>
   695	';

From template:

	<td>{{mail}}</td>

For now we are fixing this by converting null values to empty string (to silence the warnings) but that feels bad.
Probably an easy fix in the library, right?

[scr4bble]

@bobthecow any thoughts on this? Should I make a PR with fix?