Bug: minitrace-macro expansion adds empty {} and <>

Legacy `minitrace-macro` code converts 
```rust
fn f() {}
```
into
```rust
fn f<>() {
    let __guard = minitrace::local::LocalSpan::enter_with_local_parent("f");
    {}
}
```
There are two issues:
1. The `__guard` should be inside the block expression - correct?
2. The superfluous `<>`

This arose in the course of writing unit tests for issue #113 