### CI Link

https://gradle-enterprise.elastic.co/s/yp56iltyyinha

### Repro line

./gradlew ':x-pack:plugin:ilm:qa:multi-node:javaRestTest' --tests "org.elasticsearch.xpack.ilm.ChangePolicyforIndexIT.testChangePolicyForIndex" -Dtests.seed=8825024EAE3089DD -Dtests.locale=en-PH -Dtests.timezone=Asia/Karachi -Druntime.java=16

### Does it reproduce?

No

### Applicable branches

7.17 (probably all 8+)

### Failure history

_No response_

### Failure excerpt

This is rather concerning
```
java.lang.AssertionError: policy step registry cache failed sanity check
	at org.elasticsearch.xpack.ilm.PolicyStepsRegistry.getStep(PolicyStepsRegistry.java:378) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleRunner.getCurrentStep(IndexLifecycleRunner.java:108) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleRunner.runPolicyAfterStateChange(IndexLifecycleRunner.java:430) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleService.triggerPolicies(IndexLifecycleService.java:416) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleService.clusterChanged(IndexLifecycleService.java:319) ~[?:?]
	at org.elasticsearch.cluster.service.ClusterApplierService.callClusterStateListener(ClusterApplierService.java:573) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.callClusterStateListeners(ClusterApplierService.java:559) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.applyChanges(ClusterApplierService.java:519) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.runTask(ClusterApplierService.java:428) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.access$000(ClusterApplierService.java:56) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService$UpdateTask.run(ClusterApplierService.java:154) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:718) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:262) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:225) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630) ~[?:?]
	at java.lang.Thread.run(Thread.java:831) [?:?]
[2022-03-15T09:34:09.657115416Z] [BUILD] Stopping node
```
x-pack/plugin/ilm/src/test/java/org/elasticsearch/xpack/ilm/PolicyStepsRegistryTests.java
Step.StepKey errorStepKey = new Step.StepKey(randomAlphaOfLengthBetween(1, 10), randomAlphaOfLengthBetween(1, 10), ErrorStep.NAME);
        Step expectedStep = new ErrorStep(errorStepKey);
        Index index = new Index("test", "uuid");
        Map<Index, List<Step>> indexSteps = Collections.singletonMap(index, Collections.singletonList(expectedStep));
        PolicyStepsRegistry registry = new PolicyStepsRegistry(null, null, null, NamedXContentRegistry.EMPTY, null, null);
        Step actualStep = registry.getStep(emptyMetadata(index), errorStepKey);
        assertThat(actualStep, equalTo(expectedStep));
    }
    public void testGetStepUnknownPolicy() {
        PolicyStepsRegistry registry = new PolicyStepsRegistry(null, null, null, NamedXContentRegistry.EMPTY, null, null);
        IllegalArgumentException e = expectThrows(
            IllegalArgumentException.class,
            () -> registry.getStep(emptyMetadata(new Index("test", "uuid")), MOCK_STEP_KEY)
        );
        assertThat(
            e.getMessage(),
            containsString(
                "failed to retrieve step {\"phase\":\"mock\",\"action\":\"mock\",\"name\":\"mock\"}" + " as index [test] has no policy"
            )
        );
    }
    public void testGetStepForIndexWithNoPhaseGetsInitializationStep() {
        Client client = mock(Client.class);
        Mockito.when(client.settings()).thenReturn(Settings.EMPTY);
        LifecyclePolicy policy = LifecyclePolicyTests.randomTimeseriesLifecyclePolicy("policy");
        LifecyclePolicyMetadata policyMetadata = new LifecyclePolicyMetadata(policy, Collections.emptyMap(), 1, randomNonNegativeLong());
        IndexMetadata indexMetadata = IndexMetadata.builder("test")
            .settings(
                Settings.builder()
                    .put("index.number_of_shards", 1)