patch 8.1.0618: term_getjob() does not return v:null as documented

Problem:    term_getjob() does not return v:null as documented.
Solution:   Do return v:null. (Damien)  Add a test.
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index d99a925..220b1d1 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -1714,3 +1714,8 @@
   call Stop_shell_in_terminal(bufnr)
   exe bufnr . 'bwipe'
 endfunc
+
+func Test_terminal_no_job()
+  let term = term_start('false', {'term_finish': 'close'})
+  call WaitForAssert({-> assert_equal(v:null, term_getjob(term)) })
+endfunc