Adapt to MockClientHttpResponse no longer accepting a null body

spring-boot-devtools/src/test/java/org/springframework/boot/devtools/test/MockClientHttpRequestFactory.java

@@ -118,8 +118,8 @@ public class MockClientHttpRequestFactory implements ClientHttpRequestFactory {
 		}
 
 		public ClientHttpResponse asHttpResponse(AtomicLong seq) {
-			MockClientHttpResponse httpResponse = new MockClientHttpResponse(this.payload,
-					this.status);
+			MockClientHttpResponse httpResponse = new MockClientHttpResponse(
+					this.payload == null ? new byte[0] : this.payload, this.status);
 			waitForDelay();
 			if (this.payload != null) {
 				httpResponse.getHeaders().setContentLength(this.payload.length);