Returns always true predicate if predicates are empty.

This handles a rare case where a user bypasses the many checks to ensure that the predicates aren't empty.

Fixes gh-1915

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/route/RouteDefinitionRouteLocator.java

@@ -241,6 +241,10 @@ public class RouteDefinitionRouteLocator
 	private AsyncPredicate<ServerWebExchange> combinePredicates(
 			RouteDefinition routeDefinition) {
 		List<PredicateDefinition> predicates = routeDefinition.getPredicates();
+		if (predicates == null || predicates.isEmpty()) {
+			// this is a very rare case, but possible, just match all
+			return AsyncPredicate.from(exchange -> true);
+		}
 		AsyncPredicate<ServerWebExchange> predicate = lookup(routeDefinition,
 				predicates.get(0));
 

spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/handler/RoutePredicateHandlerMappingEmptyPredicatesTests.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2013-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.gateway.handler;
+
+import java.net.URI;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import reactor.core.publisher.Flux;
+
+import org.springframework.boot.SpringBootConfiguration;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.cloud.gateway.route.RouteDefinition;
+import org.springframework.cloud.gateway.route.RouteDefinitionLocator;
+import org.springframework.cloud.gateway.test.BaseWebClientTests;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Import;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.junit4.SpringRunner;
+
+import static org.springframework.boot.test.context.SpringBootTest.WebEnvironment.RANDOM_PORT;
+
+@RunWith(SpringRunner.class)
+@SpringBootTest(webEnvironment = RANDOM_PORT, properties = {})
+@DirtiesContext
+public class RoutePredicateHandlerMappingEmptyPredicatesTests extends BaseWebClientTests {
+
+	@Test
+	public void requestsToManagementPortReturn404() {
+	}
+
+	@EnableAutoConfiguration
+	@SpringBootConfiguration
+	@Import(DefaultTestConfig.class)
+	public static class TestConfig {
+
+		@Bean
+		RouteDefinitionLocator myRouteDefinitionLocator() {
+			RouteDefinition definition = new RouteDefinition();
+			definition.setId("empty_route");
+			definition.setUri(URI.create("https://example"));
+			return () -> Flux.just(definition);
+		}
+
+	}
+
+}