mistrust: Canonicalise prefix from Some("") to None

This allows us to add the proper default example to the arti example
config file.

crates/arti/src/arti-example-config.toml

@@ -97,6 +97,7 @@
 # (This is not the default.)
 #
 #     ignore_prefix = "/home/"
+#ignore_prefix = ""
 
 # Replacement values for consensus parameters.  This is an advanced option
 # and you probably should leave it alone. Not all parameters are supported.

crates/fs-mistrust/src/lib.rs

@@ -358,6 +358,7 @@ pub struct Mistrust {
 #[allow(clippy::option_option)]
 fn canonicalize_opt_prefix(prefix: &Option<Option<PathBuf>>) -> Result<Option<PathBuf>> {
     match prefix {
+        Some(Some(path)) if path.as_os_str().is_empty() => Ok(None),
         Some(Some(path)) => Ok(Some(
             path.canonicalize()
                 .map_err(|e| Error::inspecting(e, path))?,