BaseUrlUriBuilderFactory does not use base URL with an empty URI

`BaseUrlUriBuilderFactory#uriString("")` does not use the base URL as it expects the URI to start with `/`.

Courtesy of @wilkinsona, we should mimic https://github.com/spring-projects/spring-framework/blob/22ecab5aab80eb09a3e099264fb4bece8f860558/spring-web/src/main/java/org/springframework/web/util/DefaultUriBuilderFactory.java#L290-L292