Undertow, like Tomcat, uses "" for the context path of the root
context. Previously, the Undertow deployment was being configured with
"/" for the root context. This was leading to a silent failure in
AsyncContextImpl.dispatch when it failed to look up the deployment
manager for the current request.

This commit updates UndertowEmbeddedServletContainerFactory to use the
correct context path (an empty String) for the root context.

Fixes gh-2365