Deprecated functionality: strlen() in Tablerate.php

In order to reproduce this issue use a table rate in your test environment then follow the checkout steps to the end. Starting with PHP 8.1 to 8.3 you will get this error in the OM log file.

```
Deprecated functionality: strlen(): Passing null to parameter #1 ($string) of type string is deprecated  in /var/www/html/app/code/core/Mage/Shipping/Model/Resource/Carrier/Tablerate.php on line 131
```

My approach is using strlen($variable ?? '') but it could be strlen((string) $variabile) too. I am open to opinions.

app/code/core/Mage/Shipping/Model/Resource/Carrier/Tablerate.php

@@ -128,7 +128,7 @@ public function getRate(Mage_Shipping_Model_Rate_Request $request)
 
         $i = 0;
         $postcode = $request->getDestPostcode();
-        while (strlen($postcode) > 1) {
+        while (strlen($postcode ?? '') > 1) {
             $i++;
             $postcode = substr($postcode, 0, -1);
             $bind[':wildcard_postcode_' . $i] = "{$postcode}*";