Refine null-safety in spring-jms

See gh-32475

spring-jms/src/main/java/org/springframework/jms/config/JmsListenerContainerParser.java

@@ -24,6 +24,7 @@ import org.springframework.beans.PropertyValues;
 import org.springframework.beans.factory.config.RuntimeBeanReference;
 import org.springframework.beans.factory.support.RootBeanDefinition;
 import org.springframework.beans.factory.xml.ParserContext;
+import org.springframework.lang.Nullable;
 import org.springframework.util.StringUtils;
 
 /**
@@ -56,6 +57,7 @@ class JmsListenerContainerParser extends AbstractListenerContainerParser {
 
 
 	@Override
+	@Nullable
 	protected RootBeanDefinition createContainerFactory(String factoryId, Element containerEle, ParserContext parserContext,
 			PropertyValues commonContainerProperties, PropertyValues specificContainerProperties) {
 

spring-jms/src/main/java/org/springframework/jms/connection/CachingConnectionFactory.java

@@ -240,6 +240,7 @@ public class CachingConnectionFactory extends SingleConnectionFactory {
 	 * Checks for a cached Session for the given mode.
 	 */
 	@Override
+	@Nullable
 	protected Session getSession(Connection con, Integer mode) throws JMSException {
 		if (!this.active) {
 			return null;

spring-jms/src/main/java/org/springframework/jms/support/JmsUtils.java

@@ -244,6 +244,7 @@ public abstract class JmsUtils {
 	 * @return the descriptive message String
 	 * @see jakarta.jms.JMSException#getLinkedException()
 	 */
+	@Nullable
 	public static String buildExceptionMessage(JMSException ex) {
 		String message = ex.getMessage();
 		Exception linkedEx = ex.getLinkedException();