Fix elasticsearch audit finder in drive for hot reload

****** chain ******
Name: NuxeoDrive.GetChangeSummary
Exception: OperationException
Caught error: Failed to invoke operation NuxeoDrive.GetChangeSummary
Caused by: org.nuxeo.ecm.automation.OperationException: Failed to invoke operation NuxeoDrive.GetChangeSummary
****** Hierarchy calls ******
        org.nuxeo.drive.operations.NuxeoDriveGetChangeSummary

                at org.nuxeo.ecm.automation.core.trace.TracerFactory.onTrace(TracerFactory.java:198)
                at org.nuxeo.ecm.automation.core.trace.Tracer.popContext(Tracer.java:73)
                at org.nuxeo.ecm.automation.core.trace.Tracer.onChainExit(Tracer.java:86)
                at org.nuxeo.ecm.automation.core.impl.OperationChainCompiler$CompiledChainImpl.invoke(OperationChainCompiler.java:206)
                ... 106 more
Caused by: java.lang.NullPointerException
        at org.nuxeo.elasticsearch.client.ESTransportClient.search(ESTransportClient.java:201)
        at org.nuxeo.drive.elasticsearch.ESAuditChangeFinder.getUpperBound(ESAuditChangeFinder.java:278)
        at org.nuxeo.drive.service.impl.NuxeoDriveManagerImpl.getChangeSummary(NuxeoDriveManagerImpl.java:374)
        at org.nuxeo.drive.service.impl.NuxeoDriveManagerImpl.getChangeSummaryIntegerBounds(NuxeoDriveManagerImpl.java:359)
        at org.nuxeo.drive.operations.NuxeoDriveGetChangeSummary.run(NuxeoDriveGetChangeSummary.java:71)
        at sun.reflect.GeneratedMethodAccessor294.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.nuxeo.ecm.automation.core.impl.InvokableMethod.doInvoke(InvokableMethod.java:166)
        at org.nuxeo.ecm.automation.core.impl.InvokableMethod.invoke(InvokableMethod.java:178)
        ... 108 more