printf: Add integration test for %q ''

Author: sargas

tests/by-util/test_printf.rs

@@ -222,6 +222,11 @@ fn sub_q_string_special_non_printable() {
         .stdout_only("non-printable: test~");
 }
 
+#[test]
+fn sub_q_string_empty() {
+    new_ucmd!().args(&["%q", ""]).succeeds().stdout_only("''");
+}
+
 #[test]
 fn sub_char() {
     new_ucmd!()