Add assert.Empty

[skelouse]

Fixes #250

[CLAassistant]

All committers have signed the CLA.

[dolmen]

👍 for cmp.Zero because the zero value of a type is well-defined in Go.
👎 for cmp.Empty because empty is more ambiguous (ex nil slice vs slice of len 0 vs slice of capacity 0).

[skelouse]

Initial issue was not accepted, and generics will handle this better if someone wants to implement something similar in the future.