Fix a corner case when printing unaccounted time
Fix a corner case when printing unaccounted time

Previously `ra_prof` wouldn't actually print the unaccounted time in
some cases.

We would print, for instance, this:
```
    5ms - foo
        2ms - bar
```
instead of:
```
    5ms - foo
        2ms - bar
        3ms - ???
```

The fix is to properly handle the case when an entry has 0 children
instead of using the `last` variable.

Signed-off-by: Michal Terepeta <369df49275e5bc251c4c73f6250cec386e4069c3@gmail.com>

rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer