Describe the bug
Validation failed error when merging an order with fractional order item quantity if that product has zero available inventory.

To Reproduce
Steps to reproduce the behavior:

1. Place an order for 1 lb Grape
2. Place a second order for 1 lb Apple
3. Update second order to be 0.5 lb Apple
4. Go to products page and ensure Apple inventory = 0
5. Merge second order to first order
6. See error Validation failed: Unfortunately, there are only 0 Apples available.

Expected behavior
Merges fine

Additional context
Rollbar item: https://rollbar.com/LocalOrbit/Local-Orbit/items/698/occurrences/114031853146/