added float conversion, avoided error/warning

Preparations

1. ensure that PHP version 7.x is used
2. ensure that Measures System US/UK is selected (it is default for clean install)

Test

1. login to the adn.console
2. go to the Website & Content → Products section
3. press New Product button the Products grid's toolbar, choose Tangible
4. enter values for all required fields (Title, SKU)
5. enter weight 1 in the pounds field, left ounces field empty
6. press Save button in the Adding Product... form's toolbar
7. confirm, that product is created and it's weight is 1 pound and 0 ounces
8. press New Product button the Products grid's toolbar, choose Tangible
9. enter values for all required fields (Title, SKU)
10. enter weight 1 in the ounces field, left pounds field empty
11. press Save button in the Adding Product... form's toolbar
12. confirm, that product is created and it's weight is 0 pounds and 1 ounce
13. go to the Logs & Reports → System Log section
14. confirm, that no “A non-numeric value encountered” warning records in the System Log created while testing