Warn when an import list is empty
For a given file
```rust
use std::*;
use std::{};
```
output the following warnings
```
warning: unused import: `use std::{};`, #[warn(unused_imports)] on by default
--> file.rs:2:1
|
2 | use std::{};
| ^^^^^^^^^^^^
warning: unused import: `std::*;`, #[warn(unused_imports)] on by default
--> file.rs:1:5
|
1 | use std::*;
| ^^^^^^^
```