Date times with empty label don't work

**Describe the bug**
The delivery time periods should have a label, instead they'll not work. We need to fix it, in case just the delivery time is sufficient as select label.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to `WooCommerce > Delivery days`
2. Create an empty label delivery time
3. Add a product to the cart and go to checkout
