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_execute_func.vim b/src/testdir/test_execute_func.vim
index 235107c..e02b08e 100644
--- a/src/testdir/test_execute_func.vim
+++ b/src/testdir/test_execute_func.vim
@@ -132,7 +132,7 @@
   unlet xyz
 endfunc
 
-func Test_execute_null()
+func Test_execute_func_with_null()
   call assert_equal("", execute(test_null_string()))
   call assert_equal("", execute(test_null_list()))
   call assert_fails('call execute(test_null_dict())', 'E731:')
@@ -143,3 +143,5 @@
     call assert_fails('call execute(test_null_channel())', 'E908:')
   endif
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab