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-core/src/main/java/org/springframework/core/CoroutinesUtils.java

@@ -123,7 +123,7 @@ public abstract class CoroutinesUtils {
 					}
 					return KCallables.callSuspendBy(function, argMap, continuation);
 				})
-				.filter(result -> !Objects.equals(result, Unit.INSTANCE))
+				.filter(result -> result != Unit.INSTANCE)
 				.onErrorMap(InvocationTargetException.class, InvocationTargetException::getTargetException);
 
 		KClassifier returnType = function.getReturnType().getClassifier();