patch 8.2.0101: crash when passing null object to ":echomsg"

Problem:    Crash when passing null object to ":echomsg".
Solution:   Check for NULL pointer. (Yasuhiro Matsumoto, closes #5460)
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 6bb553c..3e7d63c 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -268,3 +268,13 @@
   call term_sendkeys(buf, ':q!')
   call StopVimInTerminal(buf)
 endfunc
+
+func Test_null()
+  echom test_null_list()
+  echom test_null_dict()
+  echom test_null_blob()
+  echom test_null_job()
+  echom test_null_string()
+  echom test_null_channel()
+  echom test_null_partial()
+endfunc