subscriber: fix leading comma with Pretty formatter

PR #1661 introduced a regression with the `Pretty` formatter: the
`PrettyVisitor` type was [accidentally changed][1] from being
constructed with `is_empty: true` to being constructed with `is_empty:
false` This means that when visiting a set of span fields, we emit a
leading `, ` _before_ the first field, which looks quite bad.

This branch changes it back, and now the output looks nice again. :)

[1]: 937c5d7#diff-a27a4c3564a0c2f1b7af32be0f9eec25ddfbe8b4c2be8d74e84d874b919b393bR227

tracing-subscriber/src/fmt/format/pretty.rs

@@ -227,7 +227,7 @@ where
 
 impl<'writer> FormatFields<'writer> for Pretty {
     fn format_fields<R: RecordFields>(&self, writer: Writer<'writer>, fields: R) -> fmt::Result {
-        let mut v = PrettyVisitor::new(writer, false);
+        let mut v = PrettyVisitor::new(writer, true);
         fields.record(&mut v);
         v.finish()
     }