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/eval.c b/src/eval.c
index 43c2008..7130620 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -4528,8 +4528,9 @@
 	case VAR_LIST:
 	    if (tv->vval.v_list == NULL)
 	    {
+		// NULL list is equivalent to empty list.
 		*tofree = NULL;
-		r = NULL;
+		r = (char_u *)"[]";
 	    }
 	    else if (copyID != 0 && tv->vval.v_list->lv_copyID == copyID
 		    && tv->vval.v_list->lv_len > 0)
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 32ffb48..ac91aa0 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -76,6 +76,7 @@
   call assert_equal("\n12345", execute(':echomsg 12345'))
   call assert_equal("\n[]", execute(':echomsg []'))
   call assert_equal("\n[1, 2, 3]", execute(':echomsg [1, 2, 3]'))
+  call assert_equal("\n[1, 2, []]", execute(':echomsg [1, 2, test_null_list()]'))
   call assert_equal("\n{}", execute(':echomsg {}'))
   call assert_equal("\n{'a': 1, 'b': 2}", execute(':echomsg {"a": 1, "b": 2}'))
   if has('float')
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,