fix: use bare return instead of return undef in list-context methods

Expat/Expat.pm

@@ -314,7 +314,7 @@ sub expand_ns_prefix {
         return ( defined($stack) and @$stack ) ? $stack->[-1] : undef;
     }
 
-    return undef;
+    return;
 }
 
 sub current_ns_prefixes {
@@ -676,7 +676,7 @@ sub children {
     if ( defined $children ) {
         return @$children;
     }
-    return undef;
+    return;
 }
 
 sub asString {

t/return_undef_list_context.t

@@ -0,0 +1,63 @@
+use strict;
+use warnings;
+
+use Test::More tests => 6;
+use XML::Parser;
+
+# ContentModel::children() should return an empty list when there are
+# no children (EMPTY/ANY models), not (undef).  Using "return undef"
+# in a list-returning method is a classic Perl footgun: it produces a
+# one-element list containing undef instead of an empty list.
+#
+# This means code like:
+#   my @kids = $model->children;
+#   if (@kids) { ... }  # incorrectly true for EMPTY models
+# would silently process undef as a child element.
+
+my %models;
+
+my $p = XML::Parser->new(
+    Handlers => {
+        Element => sub { $models{ $_[1] } = $_[2] },
+    },
+);
+
+$p->parse(<<'XML');
+<?xml version="1.0"?>
+<!DOCTYPE doc [
+  <!ELEMENT empty EMPTY>
+  <!ELEMENT any ANY>
+  <!ELEMENT seq (a,b)>
+]>
+<doc/>
+XML
+
+# EMPTY model has no children
+{
+    my $m = $models{empty};
+    ok( $m->isempty, 'EMPTY model detected' );
+
+    my @kids = $m->children;
+    is( scalar @kids, 0,
+        'children() returns empty list for EMPTY model' );
+}
+
+# ANY model has no children
+{
+    my $m = $models{any};
+    ok( $m->isany, 'ANY model detected' );
+
+    my @kids = $m->children;
+    is( scalar @kids, 0,
+        'children() returns empty list for ANY model' );
+}
+
+# SEQ model has children — should still work
+{
+    my $m = $models{seq};
+    ok( $m->isseq, 'SEQ model detected' );
+
+    my @kids = $m->children;
+    is( scalar @kids, 2,
+        'children() returns correct count for SEQ model' );
+}