patch 8.2.4946: Vim9: some code not covered by tests

Problem:    Vim9: some code not covered by tests.
Solution:   Add a few more test cases.  Remove dead code.
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 6fc7d06..318968b 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -78,6 +78,7 @@
 
 def Test_add()
   v9.CheckDefAndScriptFailure(['add({}, 1)'], ['E1013: Argument 1: type mismatch, expected list<any> but got dict<unknown>', 'E1226: List or Blob required for argument 1'])
+  v9.CheckDefAndScriptFailure(['add([])'], 'E119:')
   v9.CheckDefExecFailure([
         'var ln: list<number> = [1]',
         'add(ln, "a")'],