Retrieving a deleted TD causes either a NullPointerException or an empty payload to be returned

[vcharpenay]

If accepted content type is an RDF format, an empty payload is returned.

See stack trace when accepted content type is application/td+json:

java.lang.NullPointerException
	at com.github.jsonldjava.core.JsonLdApi.filterNodes(JsonLdApi.java:1697)
	at com.github.jsonldjava.core.JsonLdApi.frame(JsonLdApi.java:1402)
	at com.github.jsonldjava.core.JsonLdApi.frame(JsonLdApi.java:1353)
	at com.github.jsonldjava.core.JsonLdProcessor.frame(JsonLdProcessor.java:330)
	at com.github.jsonldjava.core.JsonLdProcessor$frame.call(Unknown Source)
	at org.eclipse.thingweb.directory.rdf.TDSerializer.writeContent(TDSerializer.groovy:113)
	at org.eclipse.thingweb.directory.ResourceSerializer$writeContent$0.call(Unknown Source)
	at org.eclipse.thingweb.directory.ResourceManager.get(ResourceManager.groovy:146)
	at org.eclipse.thingweb.directory.servlet.RegistrationHandleServlet.doGet(RegistrationHandleServlet.java:57)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
	at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:812)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1669)
	at org.eclipse.thingweb.directory.filter.ContentNegotiationFilter.doFilter(ContentNegotiationFilter.java:127)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
	at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:577)
	at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
	at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
	at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
	at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
	at org.eclipse.jetty.server.Server.handle(Server.java:499)
	at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:311)
	at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:258)
	at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
	at java.lang.Thread.run(Thread.java:745)