[current-page:query:?] should fall back to empty string

[current-page:query:?] tokens (for example [current-page:query:key]) should fall back to empty string if there is not query 'key' on the current page.

Example:

http://example.com/?key=123 should return '123'
http://example.com/ should return ''

Currently [current-page:query:key] produces '[current-page:query:key]' instead of empty string.