patch 8.2.0618: echoing a null list results in no output

Problem:    Echoing a null list results in no output. (Yegappan Lakshmanan)
Solution:   Return "[]" instead of NULL in echo_string_core().

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    618,
 /**/
     617,
 /**/