Fix empty context path warning when using Jetty with WebFlux

Previously, the context path was set to an empty string. The led to
Jetty logging a warning about an empty context path and then using
/ instead.

This commit avoids the warning while leaving the context path's end
result unchanged by setting the context path to /.

Closes gh-17399

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/jetty/JettyReactiveWebServerFactory.java

@@ -173,7 +173,7 @@ public class JettyReactiveWebServerFactory extends AbstractReactiveWebServerFact
 		server.addConnector(createConnector(address, server));
 		ServletHolder servletHolder = new ServletHolder(servlet);
 		servletHolder.setAsyncSupported(true);
-		ServletContextHandler contextHandler = new ServletContextHandler(server, "", false, false);
+		ServletContextHandler contextHandler = new ServletContextHandler(server, "/", false, false);
 		contextHandler.addServlet(servletHolder, "/");
 		server.setHandler(addHandlerWrappers(contextHandler));
 		JettyReactiveWebServerFactory.logger.info("Server initialized with port: " + port);