Rollup merge of #38085 - estebank:empty-import-list-fix-38012, r=jseyfried

Rollup merge of #38085 - estebank:empty-import-list-fix-38012, r=jseyfried

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::*;
  |     ^^^^^^^
```