Defensive resolution of getParameterType (actually never returning null)

Covers use of ResolvableType in overridden getParameterType variants.

Issue: SPR-16072

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/InvocableHandlerMethod.java

@@ -280,7 +280,7 @@ public class InvocableHandlerMethod extends HandlerMethod {
 				return this.returnValue.getClass();
 			}
 			if (!ResolvableType.NONE.equals(this.returnType)) {
-				return this.returnType.resolve();
+				return this.returnType.resolve(Object.class);
 			}
 			return super.getParameterType();
 		}