Explicitly treat null as an empty string when interpolating.

Fixes a deprecation warning on PHP 8.1+, and saves us some unnecessary escaping calls!

Thanks for your help @scr4bble :)

Fixes #383

src/Mustache/Compiler.php

@@ -506,7 +506,7 @@ class Mustache_Compiler
 
     const VARIABLE = '
         $value = $this->resolveValue($context->%s(%s), $context);%s
-        $buffer .= %s%s;
+        $buffer .= %s($value === null ? \'\' : %s);
     ';
 
     /**