Add support for empty router in RouterFunctionDsl

Issue: SPR-17247

spring-webflux/src/main/kotlin/org/springframework/web/reactive/function/server/RouterFunctionDsl.kt

@@ -546,7 +546,12 @@ open class RouterFunctionDsl(private val init: RouterFunctionDsl.() -> Unit) : (
 	 */
 	override fun invoke(): RouterFunction<ServerResponse> {
 		init()
-		return routes.reduce(RouterFunction<ServerResponse>::and)
+		return if (routes.isEmpty()) {
+			RouterFunction<ServerResponse> { Mono.empty() }
+		}
+		else {
+			routes.reduce(RouterFunction<ServerResponse>::and)
+		}
 	}
 
 }

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/RouterFunctionDslTests.kt

@@ -120,6 +120,11 @@ class RouterFunctionDslTests {
 				.verifyComplete()
 	}
 
+	@Test
+	fun emptyRouter() {
+		router { }
+	}
+
 
 	private fun sampleRouter() = router {
 		(GET("/foo/") or GET("/foos/")) { req -> handle(req) }