patch 8.2.3381: crash when using NULL list with sign functions

Problem:    Crash when using NULL list with sign functions.
Solution:   Handle a NULL list like an empty list. (issue #8260)
diff --git a/src/testdir/test_signs.vim b/src/testdir/test_signs.vim
index 738eebf..b77725c 100644
--- a/src/testdir/test_signs.vim
+++ b/src/testdir/test_signs.vim
@@ -2012,4 +2012,11 @@
   call delete("Xsign")
 endfunc
 
+func Test_sign_null_list()
+  eval test_null_list()->sign_define()
+  eval test_null_list()->sign_placelist()
+  eval test_null_list()->sign_undefine()
+  eval test_null_list()->sign_unplacelist()
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab