IntFlowContext: Fix missed import for @Nullable

Author: artembilan

Diff for: spring-integration-core/src/main/java/org/springframework/integration/dsl/context/IntegrationFlowContext.java

@@ -21,6 +21,7 @@
 import org.springframework.beans.factory.BeanFactoryAware;
 import org.springframework.integration.core.MessagingTemplate;
 import org.springframework.integration.dsl.IntegrationFlow;
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.MessageChannel;
 
 /**