Http(Async)Client not actually nullable, plus MethodInterceptor nullability

Issue: SPR-15720

spring-tx/src/main/java/org/springframework/transaction/interceptor/TransactionInterceptor.java

@@ -27,6 +27,7 @@ import org.aopalliance.intercept.MethodInvocation;
 
 import org.springframework.aop.support.AopUtils;
 import org.springframework.beans.factory.BeanFactory;
+import org.springframework.lang.Nullable;
 import org.springframework.transaction.PlatformTransactionManager;
 
 /**
@@ -86,6 +87,7 @@ public class TransactionInterceptor extends TransactionAspectSupport implements
 
 
 	@Override
+	@Nullable
 	public Object invoke(final MethodInvocation invocation) throws Throwable {
 		// Work out the target class: may be {@code null}.
 		// The TransactionAttributeSource should be passed the target class