Treat kotlin.Unit as void in web controllers

This commit fixes a regression introduced by gh-21139
via the usage of Kotlin reflection to invoke HTTP
handler methods. It ensures that kotlin.Unit is treated
as void by returning null.

It also polishes CoroutinesUtils to have a consistent
handling compared to the regular case, and adds related
tests to prevent future regressions.

Closes gh-31648

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/InvocableHandlerMethod.java

@@ -26,6 +26,7 @@ import java.util.Map;
 import java.util.Objects;
 import java.util.stream.Stream;
 
+import kotlin.Unit;
 import kotlin.coroutines.CoroutineContext;
 import kotlin.reflect.KFunction;
 import kotlin.reflect.KParameter;
@@ -326,7 +327,8 @@ public class InvocableHandlerMethod extends HandlerMethod {
 						}
 					}
 				}
-				return function.callBy(argMap);
+				Object result = function.callBy(argMap);
+				return (result == Unit.INSTANCE ? null : result);
 			}
 		}
 	}

spring-webflux/src/test/kotlin/org/springframework/web/reactive/result/InvocableHandlerMethodKotlinTests.kt

@@ -164,6 +164,20 @@ class InvocableHandlerMethodKotlinTests {
 		assertHandlerResultValue(result, "override")
 	}
 
+	@Test
+	fun unitReturnValue() {
+		val method = NullResultController::unit.javaMethod!!
+		val result = invoke(NullResultController(), method)
+		assertHandlerResultValue(result, null)
+	}
+
+	@Test
+	fun nullReturnValue() {
+		val method = NullResultController::nullable.javaMethod!!
+		val result = invoke(NullResultController(), method)
+		assertHandlerResultValue(result, null)
+	}
+
 
 	private fun invokeForResult(handler: Any, method: Method, vararg providedArgs: Any): HandlerResult? {
 		return invoke(handler, method, *providedArgs).block(Duration.ofSeconds(5))
@@ -186,7 +200,7 @@ class InvocableHandlerMethodKotlinTests {
 		return resolver
 	}
 
-	private fun assertHandlerResultValue(mono: Mono<HandlerResult>, expected: String) {
+	private fun assertHandlerResultValue(mono: Mono<HandlerResult>, expected: String?) {
 		StepVerifier.create(mono)
 				.consumeNextWith {
 					if (it.returnValue is Mono<*>) {
@@ -242,4 +256,14 @@ class InvocableHandlerMethodKotlinTests {
 		@Suppress("RedundantSuspendModifier")
 		suspend fun handleSuspending(@RequestParam value: String = "default") = value
 	}
+
+	class NullResultController {
+
+		fun unit() {
+		}
+
+		fun nullable(): String? {
+			return null
+		}
+	}
 }