Mitigate NPE in paging log statement

From the user's mailing list:

...when we remove the limit, as it was too less for our case, we got null pointer exception in logs so we are currently forced to use less page limit. Since the error here tries to convert to longValue...

[org.apache.activemq.artemis.core.server] AMQ222225: Sending unexpected exception to the client java.lang.NullPointerException: Cannot invoke "java.lang.Long.longValue()" because "this.pageLimitBytes" is null
  at org.apache.activemq.artemis.core.paging.impl.PagingStoreImpl.checkNumberOfPages(PagingStoreImpl.java:324) ~[artemis-server-2.28.0.jar:2.28.0]