GH-891 Ensure HTTP Request params are mapped even when body is not present

Resolves #891

spring-cloud-function-web/src/main/java/org/springframework/cloud/function/web/util/FunctionWebRequestProcessingHelper.java

@@ -85,27 +85,27 @@ public final class FunctionWebRequestProcessingHelper {
 
 	@SuppressWarnings({ "rawtypes", "unchecked" })
 	public static Publisher<?> processRequest(FunctionWrapper wrapper, Object argument, boolean eventStream) {
+		if (argument == null) {
+			argument = "";
+		}
 		FunctionInvocationWrapper function = wrapper.getFunction();
 
 		HttpHeaders headers = wrapper.getHeaders();
 
 		Message<?> inputMessage = null;
 
-		if (argument != null) {
-			MessageBuilder builder = MessageBuilder.withPayload(argument);
-			if (!CollectionUtils.isEmpty(wrapper.getParams())) {
-				builder = builder.setHeader(HeaderUtils.HTTP_REQUEST_PARAM, wrapper.getParams().toSingleValueMap());
-			}
-			inputMessage = builder.copyHeaders(headers.toSingleValueMap()).build();
+
+		MessageBuilder builder = MessageBuilder.withPayload(argument);
+		if (!CollectionUtils.isEmpty(wrapper.getParams())) {
+			builder = builder.setHeader(HeaderUtils.HTTP_REQUEST_PARAM, wrapper.getParams().toSingleValueMap());
 		}
+		inputMessage = builder.copyHeaders(headers.toSingleValueMap()).build();
 
 		if (function.isRoutingFunction()) {
 			function.setSkipOutputConversion(true);
 		}
 
-		Object input = argument == null ? "" : (argument instanceof Publisher ? Flux.from((Publisher) argument) : inputMessage);
-
-		Object result = function.apply(input);
+		Object result = function.apply(inputMessage);
 		if (function.isConsumer()) {
 			if (result instanceof Publisher) {
 				Mono.from((Publisher) result).subscribe();

spring-cloud-function-web/src/test/java/org/springframework/cloud/function/web/function/FunctionEndpointInitializerTests.java

@@ -17,6 +17,11 @@
 package org.springframework.cloud.function.web.function;
 
 import java.net.URI;
+
+import java.time.Duration;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.function.BiFunction;
 import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.Supplier;
@@ -25,7 +30,9 @@ import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
+import org.springframework.boot.SpringApplication;
 import org.springframework.boot.SpringBootConfiguration;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.test.web.client.TestRestTemplate;
 import org.springframework.cloud.function.context.FunctionRegistration;
 import org.springframework.cloud.function.context.FunctionType;
@@ -35,6 +42,14 @@ import org.springframework.context.support.GenericApplicationContext;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
 import org.springframework.util.SocketUtils;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.ResolvableType;
+import org.springframework.http.HttpEntity;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpMethod;
+import org.springframework.web.util.UriComponentsBuilder;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -56,6 +71,29 @@ public class FunctionEndpointInitializerTests {
 	public void close() throws Exception {
 		System.clearProperty("server.port");
 	}
+	
+	@SuppressWarnings({ "rawtypes", "unchecked" })
+	@Test
+	public void testEmptyBodyRequestParameters() throws Exception {
+		SpringApplication.run(BeansConfiguration.class);
+		String port = System.getProperty("server.port");
+		TestRestTemplate testRestTemplate = new TestRestTemplate();
+		Map<String, String> params = new HashMap<>();
+		params.put("fname", "Jim");
+		params.put("lname", "Lahey");
+
+		HttpHeaders headers = new HttpHeaders();
+		headers.set("Accept", "application/json");
+		HttpEntity entity = new HttpEntity(headers);
+
+		String urlTemplate = UriComponentsBuilder.fromHttpUrl("http://localhost:" + port + "/nullPayload")
+				.queryParam("fname", "Jim").queryParam("lname", "Lahey").encode().toUriString();
+
+		ResponseEntity<String> response = testRestTemplate.exchange(urlTemplate, HttpMethod.GET, entity, String.class);
+		String res = response.getBody();
+		assertThat(res).contains("Jim");
+		assertThat(res).contains("Lahey");
+	}
 
 	@Test
 	public void testNonExistingFunction() throws Exception {
@@ -144,6 +182,17 @@ public class FunctionEndpointInitializerTests {
 
 	}
 
+	@EnableAutoConfiguration
+	@Configuration
+	protected static class BeansConfiguration {
+		@Bean
+		public BiFunction<String, Map<String, Object>, Map<String, Object>> nullPayload() {
+			return (p, h) -> {
+				return h;
+			};
+		}
+	}
+
 
 	@SpringBootConfiguration
 	protected static class ApplicationConfiguration