Update StockItemValidator.php

Error message has previously been updated to misleading text and lacked the "greater".
According to validation logic, values of zero are not allowed and will throw this error.

Author: ferdiusa

app/code/Magento/CatalogInventory/Model/StockItemValidator.php

@@ -67,7 +67,7 @@ public function validate(ProductInterface $product, StockItemInterface $stockIte
             throw new LocalizedException(
                 __(
                     'The "%1" value is invalid for stock item ID. '
-                    . 'Enter either zero or a number than zero to try again.',
+                    . 'Enter either null or a number greater than zero to try again.',
                     $stockItemId
                 )
             );