patch 8.0.1391: encoding empty string to JSON sometimes gives "null"

Problem:    Encoding empty string to JSON sometimes gives "null".
Solution:   Handle NULL string as empty string. (closes #2446)
diff --git a/src/version.c b/src/version.c
index 16ba82a..af507e4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -772,6 +772,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1391,
+/**/
     1390,
 /**/
     1389,