Assert job is not null in FullClusterRestartIT (#38468)

`waitForRollUpJob` is an assertBusy that waits for the rollup job
to appear in the tasks list, and waits for it to be a certain state.

However, there was a null check around the state assertion, which meant
if the job _was_ null, the assertion would be skipped, and the
assertBusy would pass withouot an exception.  This could then lead to
downstream assertions to fail because the job was not actually ready,
or in the wrong state.

This changes the test to assert the job is not null, so the assertBusy
operates as intended.
Backport of #38218

x-pack/qa/full-cluster-restart/src/test/java/org/elasticsearch/xpack/restart/FullClusterRestartIT.java

@@ -631,9 +631,8 @@ private void assertRollUpJob(final String rollupJob) throws Exception {
         final Request getRollupJobRequest = new Request("GET", "_xpack/rollup/job/" + rollupJob);
         Map<String, Object> getRollupJobResponse = entityAsMap(client().performRequest(getRollupJobRequest));
         Map<String, Object> job = getJob(getRollupJobResponse, rollupJob);
-        if (job != null) {
-            assertThat(ObjectPath.eval("status.job_state", job), expectedStates);
-        }
+        assertNotNull(job);
+        assertThat(ObjectPath.eval("status.job_state", job), expectedStates);
 
         // check that the rollup job is started using the Tasks API
         final Request taskRequest = new Request("GET", "_tasks");
@@ -679,9 +678,8 @@ private void waitForRollUpJob(final String rollupJob, final Matcher<?> expectedS
             assertThat(getRollupJobResponse.getStatusLine().getStatusCode(), equalTo(RestStatus.OK.getStatus()));
 
             Map<String, Object> job = getJob(getRollupJobResponse, rollupJob);
-            if (job != null) {
-                assertThat(ObjectPath.eval("status.job_state", job), expectedStates);
-            }
+            assertNotNull(job);
+            assertThat(ObjectPath.eval("status.job_state", job), expectedStates);
         }, 30L, TimeUnit.SECONDS);
     }