patch 8.2.0634: crash with null partial and blob

Problem:    Crash with null partial and blob.
Solution:   Check for NULL pointer.  Add more tests. (Yegappan Lakshmanan,
            closes #5984)
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index ac91aa0..84808ef 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -305,9 +305,12 @@
   echom test_null_dict()
   echom test_null_blob()
   echom test_null_string()
+  echom test_null_function()
   echom test_null_partial()
   if has('job')
     echom test_null_job()
     echom test_null_channel()
   endif
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab