null-waarden afvangen in Beheer en datumhelpers (PHP 8.1 deprecations)

- Beheer: systemnorm null -> '' voor str_replace (zelfde resultaat als PHP 7)
- mysql2date/date2mysql: strtotime() krijgt '' i.p.v. null

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

application/helpers/text_helper.php

@@ -15,11 +15,11 @@
 	}
 
 	function date2mysql($date){
-		return date('Y-m-d', strtotime($date));
+		return date('Y-m-d', strtotime($date ?? ''));
 	}
 
 	function mysql2date($date){
-		return date('d-m-Y', strtotime($date));
+		return date('d-m-Y', strtotime($date ?? ''));
 	}
 	function mysql2bedrag($bedrag){
 		$bedrag2=number_format((float)$bedrag,2,",",".");