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/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,