Clarify that #[repr(transparent)] is valid without a non-ZST field

The following is valid, but the prior language suggests it is not:

```rust
#[repr(transparent)]
enum Banana {
    Phone,
}

#[repr(transparent)]
struct Blooey(());

#[repr(transparent)]
struct TheVicePresidentDoesNotHaveUnilateralPowerOverFederalElections;
```

Of note: it's still invalid to have a zero-sized field in a transparent
struct that has an alignment requirement higher than 1:
```rust
#[repr(align(2))]
struct Inner;

// error[E0691]: zero-sized field in transparent struct has alignment
//               larger than 1
#[repr(transparent)]
struct Fizz(Inner);
```

Author: kupiakos

src/type-layout.md

@@ -562,11 +562,11 @@ Like all ways to create undefined behavior in safe Rust, this is a bug.
 The `transparent` representation can only be used on a [`struct`][structs]
 or an [`enum`][enumerations] with a single variant that has:
 
-- a single field with non-zero size, and
+- a single field with non-zero size, and/or
 - any number of fields with size 0 and alignment 1 (e.g. [`PhantomData<T>`]).
 
 Structs and enums with this representation have the same layout and ABI
-as the single non-zero sized field.
+as the single non-zero sized field, or size 0 and alignment 1 if not present.
 
 This is different than the `C` representation because
 a struct with the `C` representation will always have the ABI of a `C` `struct`