comm: use NUL if delimiter is empty

src/uu/comm/src/comm.rs

@@ -33,7 +33,10 @@ mod options {
 
 fn mkdelim(col: usize, opts: &ArgMatches) -> String {
     let mut s = String::new();
-    let delim = opts.value_of(options::DELIMITER).unwrap();
+    let delim = match opts.value_of(options::DELIMITER).unwrap() {
+        "" => "\0",
+        delim => delim,
+    };
 
     if col > 1 && !opts.is_present(options::COLUMN_1) {
         s.push_str(delim.as_ref());

tests/by-util/test_comm.rs

@@ -59,13 +59,12 @@ fn output_delimiter() {
         .stdout_only_fixture("ab_delimiter_word.expected");
 }
 
-#[cfg_attr(not(feature = "test_unimplemented"), ignore)]
 #[test]
-fn output_delimiter_require_arg() {
+fn output_delimiter_nul() {
     new_ucmd!()
         .args(&["--output-delimiter=", "a", "b"])
-        .fails()
-        .stderr_only("error to be defined");
+        .succeeds()
+        .stdout_only_fixture("ab_delimiter_nul.expected");
 }
 
 // even though (info) documentation suggests this is an option