Handle empty Order increment prefix (#1718)

* Handle empty Order increment prefix

A call to `strpos` with an empty needle would throw a warning pre PHP 8.0. This change will check if the prefix is not empty first before performing the call to `strpos`.

* Use the string casted prefix in `strlen` call

Co-authored-by: Ng Kiat Siong <kiatsiong.ng@gmail.com>

* Fix syntax error

Co-authored-by: Ng Kiat Siong <kiatsiong.ng@gmail.com>

app/code/core/Mage/Eav/Model/Entity/Increment/Numeric.php

@@ -45,8 +45,8 @@ public function getNextId()
 
         if (empty($last)) {
             $last = 0;
-        } else if (strpos($last, (string)$this->getPrefix()) === 0) {
-            $last = (int)substr($last, strlen($this->getPrefix()));
+        } else if (!empty($prefix = (string)$this->getPrefix()) && strpos($last, $prefix) === 0) {
+            $last = (int)substr($last, strlen($prefix));
         } else {
             $last = (int)$last;
         }