fix ls with empty string (#12086)

# Description
Fixes: #12054

It's cause by nu always add `/*` if there is a parameter in ls, then `ls
""` becomes `ls "/*"`. This pr tries to fix it by only append `/`
character if pattern is not empty.

# User-Facing Changes
NaN

# Tests + Formatting
Done

# After Submitting
NaN

---------

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

crates/nu-command/tests/commands/ls.rs

@@ -701,3 +701,13 @@ fn list_flag_false() {
         assert_eq!(actual.out, "false");
     })
 }
+
+#[test]
+fn list_empty_string() {
+    Playground::setup("ls_empty_string", |dirs, sandbox| {
+        sandbox.with_files(vec![EmptyFile("yehuda.txt")]);
+
+        let actual = nu!(cwd: dirs.test(), "ls ''");
+        assert!(actual.err.contains("does not exist"));
+    })
+}