Fix PHP warning (#9611)

CHANGELOG.md

@@ -3,7 +3,7 @@
 ## Unreleased
 
 - OAuth: Support standard authentication with short-living password received with OIDC token (#9530)
-- Fix PHP deprecation warnings (#9616)
+- Fix PHP warnings (#9616, #9611)
 
 ## Release 1.6.9
 

program/actions/mail/index.php

@@ -1189,7 +1189,11 @@ class rcmail_action_mail_index extends rcmail_action
                     $style[$idx] = $idx . ': ' . $val;
                 }
 
-                $attrs['style'] = ($attrs['style'] ? trim($attrs['style'], ';') . '; ' : '') . implode('; ', $style);
+                if (isset($attrs['style'])) {
+                    $attrs['style'] = trim($attrs['style'], '; ') . '; ' . implode('; ', $style);
+                } else {
+                    $attrs['style'] = implode('; ', $style);
+                }
             }
 
             $out = html::tag('div', $attrs, $content);