Ignore null message when introspecting resource cleanup failure

Closes gh-30597

Author: jhoeller

spring-tx/src/main/java/org/springframework/transaction/interceptor/TransactionAspectSupport.java

@@ -1067,10 +1067,11 @@ private Mono<Void> completeTransactionAfterThrowing(@Nullable ReactiveTransactio
 		 * @param ex the throwable to try to unwrap
 		 */
 		private Throwable unwrapIfResourceCleanupFailure(Throwable ex) {
-			if (ex instanceof RuntimeException &&
-					ex.getCause() != null &&
-					ex.getMessage().startsWith("Async resource cleanup failed")) {
-				return ex.getCause();
+			if (ex instanceof RuntimeException && ex.getCause() != null) {
+				String msg = ex.getMessage();
+				if (msg != null && msg.startsWith("Async resource cleanup failed")) {
+					return ex.getCause();
+				}
 			}
 			return ex;
 		}