#26, #39 - Fixed potential transitive assertion error in UrComponentsBuilder.

If a String piped into LinkBuilderSupport.slash(…) is empty, the call to UriComponentsBuilder.fromUriString(…) fails with an assertion exception. We now guard against this specific case by returning the current builder instance in case an empty String (potentially returned from the toString() method of the object handed in) would be handed to the UriComponentsBuilder.

The issue was introduced by commit 84efebc7a65afd7b5decbae054af13406ccb59c2.