For older versions of Magento, quantity field in product page is set to blank in default. In that sense that it is not considered as user friendly. Although, it’s still adding the product on the cart even if you forgot to put value on the quantity. You can solve this without any programming involves – just edit the product stock options on your magento backend configuration.
1. Go to Admin Panel -> System -> Configuration -> Inventory-> Product Stock Options
2. Now Click on Add Minimum Qty button
3. If you want to apply it in all groups, choose Customer Group: ALL GROUPS
4. Enter 1 for Minimum Qty
5. Then Save.
Hoola! That’s it. Clear your cache and try to navigate in one of your product page.