patch 8.0.0309: cannot use an empty key in json

Problem:    Cannot use an empty key in json.
Solution:   Allow for using an empty key.
diff --git a/src/version.c b/src/version.c
index 83e3b53..dbe0dcb 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    309,
+/**/
     308,
 /**/
     307,