QueueItem.getExecutable() returns null after server.getQueueItem(queueRef);

Hi,
it seems that the current implementation of the java-client-api is not able to start jobs on a current jenkins instance. We are using Jenkins 2.46.2 and the java-client-api 0.37.
Our code looks like this:
```
QueueReference queueRef = job.build(true);
QueueItem queueItem = server.getQueueItem(queueRef);
job= job.details();
...
Build build = server.getBuild(queueItem);
```
The last code line does not work and ends with a NPE:

> java.lang.NullPointerException
at com.offbytwo.jenkins.JenkinsServer.getBuild(JenkinsServer.java:810)

The same behaviour can be reproduced with:
```
JenkinsTriggerHelper triggerHelper = new JenkinsTriggerHelper(server);
BuildWithDetails build = triggerHelper.triggerJobAndWaitUntilFinished(job.getName(), true);
```