Add Nullable to API methods

Annotate the following methods in StreamsBuilderFactoryBean with Nullable.

getStreamsUncaughtExceptionHandler
getTopology
getKafkaStreams

Author: sobychacko

spring-kafka/src/main/java/org/springframework/kafka/config/StreamsBuilderFactoryBean.java

@@ -217,6 +217,7 @@ public void setStreamsUncaughtExceptionHandler(StreamsUncaughtExceptionHandler s
 	 * @return {@link StreamsUncaughtExceptionHandler}
 	 * @since 2.8.4
 	 */
+	@Nullable
 	public StreamsUncaughtExceptionHandler getStreamsUncaughtExceptionHandler() {
 		return this.streamsUncaughtExceptionHandler;
 	}
@@ -241,6 +242,7 @@ public void setCloseTimeout(int closeTimeout) {
 	 * @return {@link Topology} object
 	 * @since 2.4.4
 	 */
+	@Nullable
 	public Topology getTopology() {
 		return this.topology;
 	}
@@ -274,6 +276,7 @@ public void setCleanupConfig(CleanupConfig cleanupConfig) {
 	 * may be null if this {@link StreamsBuilderFactoryBean} hasn't been started.
 	 * @since 1.1.4
 	 */
+	@Nullable
 	public synchronized KafkaStreams getKafkaStreams() {
 		return this.kafkaStreams;
 	}