fix NPE

Caused by: java.lang.NullPointerException
	at com.liferay.portal.kernel.io.unsync.UnsyncStringReader.<init>(UnsyncStringReader.java:34)
	at com.liferay.util.xml.XMLSafeReader.<init>(XMLSafeReader.java:25)
	at com.liferay.portal.xml.SAXReaderImpl.read(SAXReaderImpl.java:437)
	at com.liferay.portal.kernel.xml.SAXReaderUtil.read(SAXReaderUtil.java:155)
	at it.smc.calendar.sync.caldav.CalDAVRequestThreadLocal.getRequestDocument(CalDAVRequestThreadLocal.java:37)
	at it.smc.calendar.sync.caldav.util.CalDAVUtil.getRequestDAVProps(CalDAVUtil.java:188)
	... 59 more