Add @Nullable WebClient in WebFluxRequestExecMH

Author: artembilan

spring-integration-webflux/src/main/java/org/springframework/integration/webflux/outbound/WebFluxRequestExecutingMessageHandler.java

@@ -106,7 +106,7 @@ public WebFluxRequestExecutingMessageHandler(Expression uriExpression) {
 	 * @param uri The URI.
 	 * @param webClient The WebClient to use.
 	 */
-	public WebFluxRequestExecutingMessageHandler(String uri, WebClient webClient) {
+	public WebFluxRequestExecutingMessageHandler(String uri, @Nullable WebClient webClient) {
 		this(new LiteralExpression(uri), webClient);
 		/*
 		 *  We'd prefer to do this assertion first, but the compiler doesn't allow it. However,
@@ -122,7 +122,7 @@ public WebFluxRequestExecutingMessageHandler(String uri, WebClient webClient) {
 	 * {@link org.springframework.beans.factory.BeanFactory}.
 	 * @param webClient The WebClient to use.
 	 */
-	public WebFluxRequestExecutingMessageHandler(Expression uriExpression, WebClient webClient) {
+	public WebFluxRequestExecutingMessageHandler(Expression uriExpression, @Nullable WebClient webClient) {
 		super(uriExpression);
 		this.webClient = (webClient == null ? WebClient.create() : webClient);
 		this.setAsync(true);