subscriber: don't print curly braces on spans with no fields

Previously, when `tracing-subscriber`'s `fmt` module printed span
contexts, curly braces were only added around a span's fields. When we
replaced the previous implementation with the new `fmt::Layer`, this
behavior was lost, and empty curly braces were added to spans without
fields.

This commit fixes this by putting back the `if` conditional that used to
guard the printing of span fields.

Signed-off-by: Eliza Weisman <[email protected]>

Author: hawkw

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

@@ -576,7 +576,10 @@ where
             let fields = &ext
                 .get::<FormattedFields<N>>()
                 .expect("Unable to find FormattedFields in extensions; this is a bug");
-            write!(f, "{}{}{}:", bold.paint("{"), fields, bold.paint("}"))
+            if !fields.is_empty() {
+                write!(f, "{}{}{}", bold.paint("{"), fields, bold.paint("}"))?;
+            }
+            f.pad(":")
         })?;
 
         if seen {