journald: fix clippy unwrap_or_default warning (#2742)

The latest Clippy emits warnings for uses of `unwrap_or_else` with
functions that return a type's `Default::default` value, such as
`.unwrap_or_else(String::new)`. Clippy would prefer us to use
`unwrap_or_default` instead, which does the same thing.

This commit fixes the lint. Personally, I don't really care about this,
but it makes the warning go away...

tracing-journald/src/lib.rs

@@ -107,7 +107,7 @@ impl Layer {
                     .and_then(|p| p.file_name())
                     .map(|n| n.to_string_lossy().into_owned())
                     // If we fail to get the name of the current executable fall back to an empty string.
-                    .unwrap_or_else(String::new),
+                    .unwrap_or_default(),
                 additional_fields: Vec::new(),
             };
             // Check that we can talk to journald, by sending empty payload which journald discards.