Fix PHP warning: "array_merge(): Expected parameter 2 to be an array, null given in sendmail.inc (#7003)

CHANGELOG

@@ -2,6 +2,7 @@ CHANGELOG Roundcube Webmail
 ===========================
 
 - Managesieve: Fix locked UI after opening filter frame (#7007)
+- Fix PHP warning: "array_merge(): Expected parameter 2 to be an array, null given in sendmail.inc (#7003)
 
 RELEASE 1.4.0
 -------------

program/steps/mail/sendmail.inc

@@ -168,7 +168,7 @@ if ($files = rcube_utils::get_input_value('_attachments', rcube_utils::INPUT_POS
         unset($COMPOSE['attachments'][$idx]);
     }
 
-    $COMPOSE['attachments'] = array_merge(array_filter($files), $COMPOSE['attachments']);
+    $COMPOSE['attachments'] = array_merge(array_filter($files), (array) $COMPOSE['attachments']);
 }
 
 // Since we can handle big messages with disk usage, we need more time to work