feat: implement `To/FromJSON Empty` (#5421)

Resolve cases when the `To/FromJSON` type classes are used with `Empty`,
e.g. in the following motivating example.

```
import Lean

structure Foo (α : Type) where
  y : Option α
deriving Lean.ToJson

#eval Lean.toJson (⟨none⟩ : Foo Empty) -- fails
```

This is a follow-up to this PR
https://github.com/leanprover/lean4/pull/5415, as suggested by
@eric-wieser. It expands on the original suggestion by also handling
`FromJSON`.

---------

Co-authored-by: Kyle Miller <kmill31415@gmail.com>