Fixed bugs for admin save base urls

[sreichel]

Description (*)

It is possible to save invalid secure/unsecure base URLs ...

• you can save empty urls (leads to error)
  An error occurred while saving this configuration: Notice: Undefined index: host in /var/www/html/app/code/core/Mage/Core/Helper/Url.php on line 175
• you can save urls with leading/trailing spaces
• you can save urls with spaces

Manual testing scenarios (*)

1. save empty base url (or js, media, ...)
2. ...

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All automated tests passed successfully (all builds are green)
• Add yourself to contributors list