join: add support for -t '\0'

src/uu/join/src/join.rs

@@ -503,6 +503,7 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         settings.separator = match value.len() {
             0 => Sep::Line,
             1 => Sep::Char(value[0]),
+            2 if value[0] == b'\\' && value[1] == b'0' => Sep::Char(0),
             _ => {
                 return Err(USimpleError::new(
                     1,

tests/by-util/test_join.rs

@@ -346,3 +346,14 @@ fn non_unicode() {
         .succeeds()
         .stdout_only_fixture("non-unicode.expected");
 }
+
+#[test]
+fn null_field_separators() {
+    new_ucmd!()
+        .arg("-t")
+        .arg("\\0")
+        .arg("non-unicode_1.bin")
+        .arg("non-unicode_2.bin")
+        .succeeds()
+        .stdout_only_fixture("null-sep.expected");
+}