VerifyCalendar fails when recurrence is an empty string

14:03:45,548 INFO  [pool-2-thread-1][VerifyProcess:81] Verifying com.liferay.portal.verify.VerifyCalendar
14:03:45,858 ERROR [pool-2-thread-1][MainServlet:217] com.liferay.portal.kernel.events.ActionException: com.liferay.portal.verify.VerifyException: org.jabsorb.serializer.UnmarshallException: couldn't parse JSON
com.liferay.portal.kernel.events.ActionException: com.liferay.portal.verify.VerifyException: org.jabsorb.serializer.UnmarshallException: couldn't parse JSON
	at com.liferay.portal.events.StartupAction.run(StartupAction.java:58)
	at com.liferay.portal.servlet.MainServlet.processStartupEvents(MainServlet.java:1306)
	at com.liferay.portal.servlet.MainServlet.init(MainServlet.java:214)
	at javax.servlet.GenericServlet.init(GenericServlet.java:160)
	at org.apache.catalina.core.StandardWrapper.initServlet(StandardWrapper.java:1266)
	at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1185)
	at org.apache.catalina.core.StandardWrapper.load(StandardWrapper.java:1080)
	at org.apache.catalina.core.StandardContext.loadOnStartup(StandardContext.java:5015)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5302)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
	at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:895)
	at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:871)
	at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:615)
	at org.apache.catalina.startup.HostConfig.deployDescriptor(HostConfig.java:649)
	at org.apache.catalina.startup.HostConfig$DeployDescriptor.run(HostConfig.java:1585)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
	at java.util.concurrent.FutureTask.run(FutureTask.java:138)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
	at java.lang.Thread.run(Thread.java:662)
Caused by: com.liferay.portal.verify.VerifyException: org.jabsorb.serializer.UnmarshallException: couldn't parse JSON
	at com.liferay.portal.verify.VerifyProcess.verify(VerifyProcess.java:87)
	at com.liferay.portal.verify.VerifyProcessUtil._verifyProcess(VerifyProcessUtil.java:113)
	at com.liferay.portal.verify.VerifyProcessUtil._verifyProcess(VerifyProcessUtil.java:78)
	at com.liferay.portal.verify.VerifyProcessUtil.verifyProcess(VerifyProcessUtil.java:47)
	at com.liferay.portal.events.StartupHelper.verifyProcess(StartupHelper.java:136)
	at com.liferay.portal.events.StartupHelperUtil.verifyProcess(StartupHelperUtil.java:52)
	at com.liferay.portal.tools.DBUpgrader.verify(DBUpgrader.java:210)
	at com.liferay.portal.events.StartupAction.doRun(StartupAction.java:182)
	at com.liferay.portal.events.StartupAction.run(StartupAction.java:52)
	... 20 more
Caused by: org.jabsorb.serializer.UnmarshallException: couldn't parse JSON
	at org.jabsorb.JSONSerializer.fromJSON(JSONSerializer.java:259)
	at com.liferay.portal.verify.VerifyCalendar.verifyRecurrence(VerifyCalendar.java:123)
	at com.liferay.portal.verify.VerifyCalendar.doVerify(VerifyCalendar.java:43)
	at com.liferay.portal.verify.VerifyProcess.verify(VerifyProcess.java:84)
	... 28 more
Caused by: org.json.JSONException: Missing value at 0 [character 1 line 1]
	at org.json.JSONTokener.syntaxError(JSONTokener.java:432)
	at org.json.JSONTokener.nextValue(JSONTokener.java:386)
	at org.jabsorb.JSONSerializer.fromJSON(JSONSerializer.java:255)
	... 31 more
Stopping the server due to unexpected startup errors