fix: Fix setup:upgrade when return value is null

Helper/Cron/ProductImages.php

@@ -207,7 +207,13 @@ class ProductImages extends AbstractHelper
             return '_';
         }
 
-        return $this->scopeConfig->getValue('importimagesproduct/pattern/delimiter', ScopeInterface::SCOPE_STORE, $storeId);
+        $value = $this->scopeConfig->getValue('importimagesproduct/pattern/delimiter', ScopeInterface::SCOPE_STORE, $storeId);
+
+        if (is_null($value)) {
+            return '_';
+        }
+
+        return $value;
     }
 
     /**