patch 8.2.0633: crash when using null partial in filter()

Problem:    Crash when using null partial in filter().
Solution:   Fix crash.  Add more tests. (Yegappan Lakshmanan, closes #5976)
diff --git a/src/testdir/test_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index 8feeb36..cd9611c 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
@@ -217,7 +217,7 @@
   call delete('Xversionscript')
 endfunc
 
-func Test_excute_null()
+func Test_execute_cmd_with_null()
   call assert_fails('execute test_null_list()', 'E730:')
   call assert_fails('execute test_null_dict()', 'E731:')
   call assert_fails('execute test_null_blob()', 'E976:')