patch 9.0.0837: append() reports failure when not appending anything

Problem:    append() reports failure when not appending anything.
Solution:   Only report failure when appending something. (closes #11498)
diff --git a/src/evalbuffer.c b/src/evalbuffer.c
index e77435b..9ce1d41 100644
--- a/src/evalbuffer.c
+++ b/src/evalbuffer.c
@@ -175,9 +175,7 @@
 	l = lines->vval.v_list;
 	if (l == NULL || list_len(l) == 0)
 	{
-	    // set proper return code
-	    if (lnum > curbuf->b_ml.ml_line_count)
-		rettv->vval.v_number = 1;	// FAIL
+	    // not appending anything always succeeds
 	    goto done;
 	}
 	CHECK_LIST_MATERIALIZE(l);