Fix NPE when the document has no core session attached

Caused by: java.lang.NullPointerException
	at org.nuxeo.ecm.core.api.localconfiguration.LocalConfigurationServiceImpl.getConfiguration(LocalConfigurationServiceImpl.java:42)
	at org.nuxeo.theme.localconfiguration.LocalThemeHelper.getLocalThemeConfig(LocalThemeHelper.java:11)
	at org.nuxeo.ecm.localconf.LocalThemeFlavor.getOutcome(LocalThemeFlavor.java:56)
	at org.nuxeo.theme.negotiation.AbstractNegotiator.negotiate(AbstractNegotiator.java:70)
	at org.nuxeo.theme.negotiation.AbstractNegotiator.getSpec(AbstractNegotiator.java:47)
	at org.nuxeo.theme.jsf.facelets.CompositionHandler.apply(CompositionHandler.java:156)
	at com.sun.faces.facelets.compiler.NamespaceHandler.apply(NamespaceHandler.java:93)
	at com.sun.faces.facelets.compiler.EncodingHandler.apply(EncodingHandler.java:87)
	at com.sun.faces.facelets.impl.DefaultFacelet.apply(DefaultFacelet.java:161)
	at com.sun.faces.application.view.FaceletViewHandlingStrategy.buildView(FaceletViewHandlingStrategy.java:965)
	at com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:99)
	at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
	at com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:219)
	at javax.faces.webapp.FacesServlet.service(FacesServlet.java:647)
	... 62 more