patch 9.0.1311: Coverity warns for using a NULL pointer

Problem:    Coverity warns for using a NULL pointer.
Solution:   Use "empty_option" instead of NULL.
diff --git a/src/version.c b/src/version.c
index a6451cb..c59a3a1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1311,
+/**/
     1310,
 /**/
     1309,