patch 8.2.3382: crash when getting the type of a NULL partial

Problem:    Crash when getting the type of a NULL partial.
Solution:   Check for NULL. (closes #8260)
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 434f477..c166a43 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -3679,6 +3679,15 @@
   if has('float')
     assert_equal('func([unknown], [unknown]): float', typename(function('pow')))
   endif
+  assert_equal('func', test_null_partial()->typename())
+  assert_equal('list<unknown>', test_null_list()->typename())
+  assert_equal('dict<unknown>', test_null_dict()->typename())
+  if has('job')
+    assert_equal('job', test_null_job()->typename())
+  endif
+  if has('channel')
+    assert_equal('channel', test_null_channel()->typename())
+  endif
 enddef
 
 def Test_undofile()