patch 8.2.0102: messages test fails in small version

Problem:    Messages test fails in small version.
Solution:   Only use test_null_job() when available.
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 3e7d63c..62a191e 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -273,8 +273,10 @@
   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()
+  if has('job')
+    echom test_null_job()
+    echom test_null_channel()
+  endif
 endfunc