patch 8.2.0539: comparing two NULL list fails

Problem:    Comparing two NULL list fails.
Solution:   Change the order of comparing two lists.
diff --git a/src/testdir/test_assert.vim b/src/testdir/test_assert.vim
index 1b1f9e5..762f12f 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -36,6 +36,9 @@
   call assert_equal(0, assert_equal(4, n))
   let l = [1, 2, 3]
   call assert_equal(0, assert_equal([1, 2, 3], l))
+  call assert_equal(test_null_list(), test_null_list())
+  call assert_equal(test_null_list(), [])
+  call assert_equal([], test_null_list())
 
   let s = 'foo'
   call assert_equal(1, assert_equal('bar', s))