Comment in empty generic arg in macro context is unexpectedly removed

Summary

Follow-up to #7181.

I tried to format this code:

macro_rules! m { ($f:ident </*c*/>) => {} }
fn main() {
    m!(Foo</*c*/>);
}

I expected to see this happen: keep the comment in the empty generic arg.

Instead, this happened: comment is removed.

Diff in /Users/joe.xu/Documents/repos/repro/foo.rs:2:
     ($f:ident </*c*/>) => {};
 }
 fn main() {
-    m!(Foo</*c*/>);
+    m!(Foo<>);
 }

rustfmt after #7181 applied.

Just run on the file.