Fix websocket connections on root servlet context installations

Currently `xcontext.getWiki().getWebAppPath(xcontext)` gives a slash when configured empty in the xwiki.cfg which is needed for a short URL installation.

xwiki.webapppath=

For the websocket-url derivation in
xwiki-platform-core/xwiki-platform-websocket/src/main/java/org/xwiki/websocket/script/WebSocketScriptService.java this gives a path like `//websocket` resulting in a not working URL and websocket connection.