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;