ES sequencer generates NPE when ES is disabled

1. install Nuxeo 9.10
2. set elasticsearch.enabled=false in nuxeo.conf
3. start nuxeo
4. observe the following in the logs:

   2018-03-26 15:10:42,678 ERROR [localhost-startStop-1] [org.nuxeo.runtime.model.impl.RegistrationInfoImpl] Component service:org.nuxeo.ecm.core.uidgen.UIDGeneratorService notification of application started failed: null
   java.lang.NullPointerException
   	at org.nuxeo.elasticsearch.ElasticSearchComponent.getClient(ElasticSearchComponent.java:240)
   	at org.nuxeo.elasticsearch.seqgen.ESUIDSequencer.init(ESUIDSequencer.java:57)
   	at org.nuxeo.ecm.core.uidgen.UIDGeneratorComponent.start(UIDGeneratorComponent.java:63)
   	at org.nuxeo.runtime.model.impl.RegistrationInfoImpl.start(RegistrationInfoImpl.java:382)
   	at org.nuxeo.runtime.model.impl.ComponentManagerImpl.startComponent(ComponentManagerImpl.java:710)
   	at org.nuxeo.runtime.model.impl.ComponentManagerImpl.startComponents(ComponentManagerImpl.java:690)
   	at org.nuxeo.runtime.model.impl.ComponentManagerImpl.start(ComponentManagerImpl.java:793)
   	at org.nuxeo.runtime.osgi.OSGiRuntimeService.startComponents(OSGiRuntimeService.java:460)
   	at org.nuxeo.runtime.osgi.OSGiRuntimeService.frameworkEvent(OSGiRuntimeService.java:475)
   	at org.nuxeo.osgi.OSGiAdapter.fireFrameworkEvent(OSGiAdapter.java:223)
   	at org.nuxeo.osgi.application.loader.FrameworkLoader.doStart(FrameworkLoader.java:226)
   	at org.nuxeo.osgi.application.loader.FrameworkLoader.start(FrameworkLoader.java:125)
   	at org.nuxeo.runtime.deployment.NuxeoStarter.start(NuxeoStarter.java:120)
   	at org.nuxeo.runtime.deployment.NuxeoStarter.contextInitialized(NuxeoStarter.java:93)
   	at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4745)
   	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5207)
   	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
   	at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:752)
   	at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:728)
   	at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:734)
   	at org.apache.catalina.startup.HostConfig.deployDescriptor(HostConfig.java:630)
   	at org.apache.catalina.startup.HostConfig$DeployDescriptor.run(HostConfig.java:1842)
   	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
   	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
   	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
   	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
   	at java.lang.Thread.run(Thread.java:745)
   

5. this does not prevent usage of Nuxeo but generates an error and cannot be circumvented through a parameter

After further investigation, sequencers are all started even though their dependencies are not. For example in UIDGeneratorComponent :
https://github.com/nuxeo/nuxeo/blob/9.10/nuxeo-core/nuxeo-core/src/main/java/org/nuxeo/ecm/core/uidgen/UIDGeneratorComponent.java#L61

    @Override
    public void start(ComponentContext context) {
        for (String name : sequencers.keySet()) {
            sequencers.get(name).init();
        }
    }

In the init method of ESUIDSequencer, it is presumed ESClient is available
https://github.com/nuxeo/nuxeo/blob/9.10/nuxeo-features/nuxeo-elasticsearch/nuxeo-elasticsearch-seqgen/src/main/java/org/nuxeo/elasticsearch/seqgen/ESUIDSequencer.java#L57
which is not due to elasticsearch.enabled=false thus leading to an NPE.