Merge pull request 'Fix an undefined array key warning in 'catchupFeed'.' (#4) from wn/tt-rss:rpc-catchupfeed-warning into master

Reviewed-on: https://git.tt-rss.org/fox/tt-rss/pulls/4

classes/rpc.php

@@ -150,7 +150,7 @@ class RPC extends Handler_Protected {
 	function catchupFeed() {
 		$feed_id = clean($_REQUEST['feed_id']);
 		$is_cat = clean($_REQUEST['is_cat']) == "true";
-		$mode = clean($_REQUEST['mode']);
+		$mode = clean($_REQUEST['mode'] ?? '');
 		$search_query = clean($_REQUEST['search_query']);
 		$search_lang = clean($_REQUEST['search_lang']);