GH-9713: Add @Nullable to IntegrationEvent.getCause()

Fixes: #9713

**Auto-cherry-pick to `6.3.x`**

spring-integration-core/src/main/java/org/springframework/integration/events/IntegrationEvent.java

@@ -43,6 +43,7 @@ public abstract class IntegrationEvent extends ApplicationEvent {
 		this.cause = cause;
 	}
 
+	@Nullable
 	public Throwable getCause() {
 		return this.cause;
 	}