nl: don't treat empty lines as headers with an empty -d argument

* src/nl.c (check_section): Add a check for a zero section delimiter
length.
* tests/nl/nl.sh: Add a test case.
* NEWS: Mention the bug fix.

Discovered by Anthropic's OSS Scanner, as bug ANT-2026-PDYYNTYZ.

NEWS

@@ -76,6 +76,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   just-moved FIFO.
   [bug introduced with SELinux support in coreutils-8.22]
 
+  'nl -d ""' no longer mistakenly treats empty lines as headers.
+  [bugs introduced in coreutils-9.11]
+
   od now exits gracefully when offsets overflow integer limits.
   [This bug was present in "the beginning".]
 

src/nl.c

@@ -408,7 +408,8 @@ check_section (void)
 {
   size_t len = line_buf.length - 1;
 
-  if (len < section_del_len || footer_del_len < section_del_len
+  if (section_del_len == 0 || len < section_del_len
+      || footer_del_len < section_del_len
       || !memeq (line_buf.buffer, section_del, section_del_len))
     return Text;
   if (len == header_del_len

tests/nl/nl.sh

@@ -124,4 +124,11 @@ EOF
 compare exp-out out || fail=1
 compare exp-err err || fail=1
 
+printf 'a\n\nb\n\nc' >inp || framework_failure_
+nl -d '' inp >out 2>err || fail=1
+printf '     1\ta\n       \n     2\tb\n       \n     3\tc\n' >exp \
+  || framework_failure_
+compare exp out || fail=1
+compare /dev/null err || fail=1
+
 Exit $fail