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().
diff --git a/src/version.c b/src/version.c
index b7958da..c080084 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    618,
+/**/
     617,
 /**/
     616,