pr: make -n/--number-lines argument optional to match GNU

GNU pr treats the -n/--number-lines argument as optional: a bare -n numbers
the lines with a 5-wide, tab-separated line number. uutils required a value,
so `pr -t -n` and `pr -t --number-lines` failed with "a value is required".
Set num_args(0..=1) so the value is optional; the existing fallback already
supplies the default numbering mode.

Author: AlejandroCoronadoN

src/uu/pr/src/pr.rs

@@ -238,6 +238,10 @@ pub fn uu_app() -> Command {
                 .long(options::NUMBER_LINES)
                 .help(translate!("pr-help-number-lines"))
                 .allow_hyphen_values(true)
+                // GNU pr treats the -n/--number-lines argument as optional and
+                // defaults to a 5-wide, tab-separated line number; without this a
+                // bare -n or --number-lines fails asking for a value.
+                .num_args(0..=1)
                 .value_name("[char][width]"),
         )
         .arg(

tests/by-util/test_pr.rs

@@ -81,6 +81,19 @@ fn test_number_lines_empty_value_is_rejected() {
         .stderr_contains("pr: '-n' extra characters or invalid number in the argument");
 }
 
+#[test]
+fn test_number_lines_without_value_numbers_lines() {
+    // GNU pr treats -n/--number-lines as optional, defaulting to a 5-wide,
+    // tab-separated line number, instead of requiring an explicit value.
+    for arg in ["-n", "--number-lines"] {
+        new_ucmd!()
+            .args(&["-t", arg])
+            .pipe_in("a\nb\n")
+            .succeeds()
+            .stdout_is("    1\ta\n    2\tb\n");
+    }
+}
+
 #[test]
 fn test_without_any_options() {
     let test_file_path = "test_one_page.log";