patch 8.2.0189: cd() with NULL argument crashes

Problem:    cd() with NULL argument crashes.
Solution:   Check for NULL. (Ken Takata, closes #5558)
diff --git a/src/testdir/test_cd.vim b/src/testdir/test_cd.vim
index 05d3877..e44b25f 100644
--- a/src/testdir/test_cd.vim
+++ b/src/testdir/test_cd.vim
@@ -101,6 +101,8 @@
   call assert_fails("call chdir('dir-abcd')", 'E472:')
   silent! let d = chdir("dir_abcd")
   call assert_equal("", d)
+  " Should not crash
+  call chdir(d)
 
   only | tabonly
   call chdir(topdir)