Fix return type for broken feeds (#3423)

Fix a rare error when an invalid feed is forced to be added again.
FreshRSS code (not upstream)

lib/SimplePie/SimplePie.php

@@ -1424,7 +1424,7 @@ class SimplePie
 			// Fetch the data via SimplePie_File into $this->raw_data
 			if (($fetched = $this->fetch_data($cache)) === true)
 			{
-				return $this->data['mtime'];
+				return empty($this->data['mtime']) ? false : $this->data['mtime'];
 			}
 			elseif ($fetched === false) {
 				return false;