..F....... [ZBX-8670] fixed undefined index error on the web scenario configuration page when no host is selected

Before the fix:
- On the web scenario configuration list when the "Dropdown first entry" option is set to "none" and no host is selected, and undefined index error would appear.
 
After the fix:
- The error no longer appears.

git-svn-id: svn://svn.zabbix.com/trunk@48507 f4e7fd02-d210-4e34-a1e0-d4093fb66cc2