patch 8.2.0634: crash with null partial and blob

Problem:    Crash with null partial and blob.
Solution:   Check for NULL pointer.  Add more tests. (Yegappan Lakshmanan,
            closes #5984)
diff --git a/src/testdir/test_cd.vim b/src/testdir/test_cd.vim
index e5c119e..da9ef08 100644
--- a/src/testdir/test_cd.vim
+++ b/src/testdir/test_cd.vim
@@ -119,6 +119,7 @@
   call assert_equal("", d)
   " Should not crash
   call chdir(d)
+  call assert_equal('', chdir([]))
 
   only | tabonly
   call chdir(topdir)