Wicket-Bootstrap #280 - Return null if a resource cannot be found

A fix for https://github.com/l0rdn1kk0n/wicket-bootstrap/issues/280
The resource finder should not throw an exception when a resource cannot be found.

The problem here was that Wicket first tries to find .min. version of the resource. If the result is null then Wicket asks for the non-min version.