fix CacheServiceImpl default descriptor handling

11:05:41,859 ERROR [RegistrationInfoImpl] Component service:org.nuxeo.ecm.core.cache.CacheService notification of application started failed: null
java.lang.NullPointerException
	at java.util.concurrent.ConcurrentHashMap.putVal(ConcurrentHashMap.java:1011)
	at java.util.concurrent.ConcurrentHashMap.put(ConcurrentHashMap.java:1006)
	at org.nuxeo.ecm.core.cache.CacheServiceImpl.startCacheDescriptor(CacheServiceImpl.java:251)
	at java.util.ArrayList.forEach(ArrayList.java:1257)
	at org.nuxeo.ecm.core.cache.CacheServiceImpl.start(CacheServiceImpl.java:229)
	at org.nuxeo.runtime.model.impl.RegistrationInfoImpl.start(RegistrationInfoImpl.java:382)
	at org.nuxeo.runtime.model.impl.ComponentManagerImpl.startComponent(ComponentManagerImpl.java:723)
	at org.nuxeo.runtime.model.impl.ComponentManagerImpl.startComponents(ComponentManagerImpl.java:703)
	at org.nuxeo.runtime.model.impl.ComponentManagerImpl.start(ComponentManagerImpl.java:806)
	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.runtime.test.RuntimeHarnessImpl.fireFrameworkStarted(RuntimeHarnessImpl.java:213)
	at org.nuxeo.runtime.test.runner.RuntimeFeature.beforeRun(RuntimeFeature.java:159)
	at org.nuxeo.runtime.test.runner.FeaturesRunner.lambda$1(FeaturesRunner.java:155)
	at org.nuxeo.runtime.test.runner.FeaturesLoader.apply(FeaturesLoader.java:126)
	at org.nuxeo.runtime.test.runner.FeaturesLoader.apply(FeaturesLoader.java:113)
	at org.nuxeo.runtime.test.runner.FeaturesRunner.beforeRun(FeaturesRunner.java:155)
	at org.nuxeo.runtime.test.runner.FeaturesRunner$BeforeClassStatement.evaluate(FeaturesRunner.java:215)
	at org.junit.rules.RunRules.evaluate(RunRules.java:20)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:538)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206)