incorrect return type noted for all_products failed lookup

#220 

Not sure if this is an error in the docs, or a regression in the actual liquid engine, but the docs and reality do not align on the return type for `all_products`

**Summary**
The docs indicate `all_products` returns `nil` on a failed lookup

**Expected**
`all_products` actually returns `empty`

**Actual**
it indicates it returns `nil`
