Minor - fix PHP warning.

main/install/install.lib.php

@@ -1320,7 +1320,7 @@ function display_database_settings_form(
         $dbUsernameForm = $_configuration['db_user'];
         $dbPassForm = $_configuration['db_password'];
         $dbNameForm = $_configuration['main_database'];
-        $dbPortForm         = $_configuration['db_port'];
+        $dbPortForm = isset($_configuration['db_port']) ? $_configuration['db_port'] : '';
 
         echo '<div class="RequirementHeading"><h2>' . display_step_sequence() .get_lang('DBSetting') . '</h2></div>';
         echo '<div class="RequirementContent">';