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/terminal.c b/src/terminal.c
index 20284af..06d470c 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -4794,11 +4794,14 @@
 {
     buf_T	*buf = term_get_buf(argvars, "term_getjob()");
 
-    rettv->v_type = VAR_JOB;
-    rettv->vval.v_job = NULL;
     if (buf == NULL)
+    {
+	rettv->v_type = VAR_SPECIAL;
+	rettv->vval.v_number = VVAL_NULL;
 	return;
+    }
 
+    rettv->v_type = VAR_JOB;
     rettv->vval.v_job = buf->b_term->tl_job;
     if (rettv->vval.v_job != NULL)
 	++rettv->vval.v_job->jv_refcount;
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
diff --git a/src/version.c b/src/version.c
index 77bfd81..1612f9d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    618,
+/**/
     617,
 /**/
     616,