GH-8754: MessageBuilder: enhance @Nullable

Fixes #8754

* `AbstractIntegrationMessageBuilder.setExpirationDate(@nullable Long expirationDate)`
* `MessageBuilder.setExpirationDate(@nullable Long expirationDate)`
* `MessageBuilder.setExpirationDate(@nullable Date expirationDate)`

**Cherry-pick to `6.1.x` & `6.0.x`**

Author: mrpiggi

spring-integration-core/src/main/java/org/springframework/integration/support/AbstractIntegrationMessageBuilder.java

@@ -44,7 +44,7 @@
  */
 public abstract class AbstractIntegrationMessageBuilder<T> {
 
-	public AbstractIntegrationMessageBuilder<T> setExpirationDate(Long expirationDate) {
+	public AbstractIntegrationMessageBuilder<T> setExpirationDate(@Nullable Long expirationDate) {
 		return setHeader(IntegrationMessageHeaderAccessor.EXPIRATION_DATE, expirationDate);
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/support/MessageBuilder.java

@@ -241,13 +241,13 @@ public MessageBuilder<T> popSequenceDetails() {
 	}
 
 	@Override
-	public MessageBuilder<T> setExpirationDate(Long expirationDate) {
+	public MessageBuilder<T> setExpirationDate(@Nullable Long expirationDate) {
 		super.setExpirationDate(expirationDate);
 		return this;
 	}
 
 	@Override
-	public MessageBuilder<T> setExpirationDate(Date expirationDate) {
+	public MessageBuilder<T> setExpirationDate(@Nullable Date expirationDate) {
 		super.setExpirationDate(expirationDate);
 		return this;
 	}