test(core): add failing test for null input

Author: marionebl

@commitlint/core/src/library/to-lines.test.js

@@ -5,6 +5,10 @@ test('should return an array for empty input', t => {
 	t.deepEqual(toLines(), []);
 });
 
+test.failing('should return an array for null input', t => {
+	t.deepEqual(toLines(null), []);
+});
+
 test('should return an array for empty string input', t => {
 	t.deepEqual(toLines(''), ['']);
 });