Add Nullable to API methods

Annotate the following methods in StreamsBuilderFactoryBean with Nullable.

getStreamsUncaughtExceptionHandler
getTopology
getKafkaStreams

Author: sobychacko

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

@@ -204,6 +204,7 @@ public void setStreamsUncaughtExceptionHandler(StreamsUncaughtExceptionHandler s
 	 * @return {@link StreamsUncaughtExceptionHandler}
 	 * @since 2.8.4
 	 */
+	@Nullable
 	public StreamsUncaughtExceptionHandler getStreamsUncaughtExceptionHandler() {
 		return this.streamsUncaughtExceptionHandler;
 	}
@@ -228,6 +229,7 @@ public void setCloseTimeout(int closeTimeout) {
 	 * @return {@link Topology} object
 	 * @since 2.4.4
 	 */
+	@Nullable
 	public Topology getTopology() {
 		return this.topology;
 	}
@@ -261,6 +263,7 @@ 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;
 	}