Fix pretty printing of empty bound lists in where-clause

Repro:

```rust
macro_rules! assert_item_stringify {
    ($item:item $expected:literal) => {
        assert_eq!(stringify!($item), $expected);
    };
}

fn main() {
    assert_item_stringify! {
        fn f<'a, T>() where 'a:, T: {}
        "fn f<'a, T>() where 'a:, T: {}"
    }
}
```

Previously this assertion would fail because rustc renders the where-clause as `where 'a, T` which is invalid syntax.

This PR makes the above assertion pass.

This bug also affects `-Zunpretty=expanded`. The intention is for that to emit syntactically valid code, but the buggy output is not valid Rust syntax.

```console
$ rustc <(echo "fn f<'a, T>() where 'a:, T: {}") -Zunpretty=expanded
#![feature(prelude_import)]
#![no_std]
#[prelude_import]
use ::std::prelude::rust_2015::*;
#[macro_use]
extern crate std;
fn f<'a, T>() where 'a, T {}
```

```console
$ rustc <(echo "fn f<'a, T>() where 'a:, T: {}") -Zunpretty=expanded | rustc -
error: expected `:`, found `,`
 --> <anon>:7:23
  |
7 | fn f<'a, T>() where 'a, T {}
  |                       ^ expected `:`
```