patch 8.2.0104: using channel or job with ":execute" has strange effects

Problem:    Using channel or job with ":execute" has strange effects.
Solution:   Give an error message for Job and Channel.
diff --git a/src/testdir/test_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index ec566da..5362755 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
@@ -216,3 +216,15 @@
   call assert_fails('source Xversionscript', 'E999:')
   call delete('Xversionscript')
 endfunc
+
+func Test_excute_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:')
+  execute test_null_string()
+  call assert_fails('execute test_null_partial()', 'E729:')
+  if has('job')
+    call assert_fails('execute test_null_job()', 'E908:')
+    call assert_fails('execute test_null_channel()', 'E908:')
+  endif
+endfunc