Deriving ArrayType for an empty `struct` causes proc macro panic

[johanpel]

with:

proc-macro panicked: called `Option::unwrap()` on a `None` value

[mbrobbel]

Ah, the derive macro assumes the field len to be non-zero for non-unit structs. We should handle these empty structs (named and unnamed) in the same way as unit structs.

Until this is fixed you can use unit structs instead of empty structs.