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)

src/version.c

@@ -771,6 +771,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1391,
 /**/
     1390,
 /**/