fix: null issue in trim method

src/PHPMailer.php

@@ -1571,7 +1571,7 @@ class PHPMailer
 
             //Validate From, Sender, and ConfirmReadingTo addresses
             foreach (['From', 'Sender', 'ConfirmReadingTo'] as $address_kind) {
-                $this->{$address_kind} = trim($this->{$address_kind});
+                $this->{$address_kind} = trim($this->{$address_kind} ?? '');
                 if (empty($this->{$address_kind})) {
                     continue;
                 }