Derive fails confusingly when there are no non-disabled variants

[PokeJofeJr4th]

The following error message is given:

error[E0392]: parameter `T` is never used
  --> strum_tests\tests\enum_map.rs:15:10
   |
15 | #[derive(EnumMap)]
   |          ^^^^^^^ unused parameter
   |
   = help: consider removing `T`, referring to it in a field, or using a marker such as `std::marker::PhantomData`
   = help: if you intended `T` to be a const parameter, use `const T: usize` instead
   = note: this error originates in the derive macro `EnumMap` (in Nightly builds, run with -Z macro-backtrace for more info)

Since type T is not exposed to the end user, this is not a useful error message. The EnumMap derive should check if the enum is empty and either return its own error explaining what's going on or remove the generic parameter.