Fixed #4217 folder_directive_snippets default value.

install/sql/incremental/upd_dev_collection.sql

+ALTER TABLE `web_domain` CHANGE `folder_directive_snippets` `folder_directive_snippets` TEXT CHARACTER SET utf8 COLLATE utf8_general_ci NULL;
\ No newline at end of file

install/sql/ispconfig3.sql

@@ -1985,7 +1985,7 @@ CREATE TABLE `web_domain` (
   `enable_pagespeed` ENUM('y','n') NOT NULL DEFAULT 'n',
   `http_port` int(11) unsigned NOT NULL DEFAULT '80',
   `https_port` int(11) unsigned NOT NULL DEFAULT '443',
-  `folder_directive_snippets` text NOT NULL,
+  `folder_directive_snippets` text,
   PRIMARY KEY  (`domain_id`),
   UNIQUE KEY `serverdomain` (  `server_id` , `ip_address`,  `domain` )
 ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;