patch 8.2.1083: crash when using reduce() on a NULL list

Problem:    Crash when using reduce() on a NULL list.
Solution:   Only access the list when not NULL.
diff --git a/src/testdir/test_listdict.vim b/src/testdir/test_listdict.vim
index 26b0e91..8a8d353 100644
--- a/src/testdir/test_listdict.vim
+++ b/src/testdir/test_listdict.vim
@@ -718,6 +718,9 @@
   call assert_fails("call reduce(g:lut, { acc, val -> EvilRemove() }, 1)", 'E742:')
   unlet g:lut
   delfunc EvilRemove
+
+  call assert_equal(42, reduce(test_null_list(), function('add'), 42))
+  call assert_equal(42, reduce(test_null_blob(), function('add'), 42))
 endfunc
 
 " splitting a string to a List using split()