Fix PulsarProperties lookupTimeout

Prefer `null` to `-1` for the default timeout.

See gh-34763

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/pulsar/PulsarProperties.java

@@ -116,7 +116,7 @@ public class PulsarProperties {
 		/**
 		 * Client lookup timeout.
 		 */
-		private Duration lookupTimeout = Duration.ofMillis(-1); // FIXME
+		private Duration lookupTimeout;
 
 		/**
 		 * Duration to wait for a connection to a broker to be established.