Fix UriHelper length calculation when path is empty

[MihaZupan]

#29448 regressed the pathBase ends with a slash && path is empty case in UriHelper.

UriHelper.BuildAbsolute("http", new HostString("example.com"), pathBase: "/", path: "");

It would end up substracting 1 from the length even if the path was empty.
The operation throws as the allocated string is too short.

[Tratcher]

Thanks for fixing the root issue.