Fix NPE hashcode in LwM2mPath

[sbernard31]

This aims to fix NullPointerException in LwM2mPath.hashcode() when objectId is null

java.lang.NullPointerException: null
	at org.eclipse.leshan.core.node.LwM2mPath.hashCode(LwM2mPath.java:336)
	at java.util.HashMap.hash(HashMap.java:339)
	at java.util.HashMap.put(HashMap.java:612)
	at org.eclipse.leshan.core.node.codec.senml.LwM2mNodeSenMLDecoder.groupByPath(LwM2mNodeSenMLDecoder.java:322)
	at org.eclipse.leshan.core.node.codec.senml.LwM2mNodeSenMLDecoder.decodeNodes(LwM2mNodeSenMLDecoder.java:130)
	at org.eclipse.leshan.core.node.codec.DefaultLwM2mDecoder.decodeNodes(DefaultLwM2mDecoder.java:178)
	at org.eclipse.leshan.server.californium.request.LwM2mResponseBuilder.decodeCompositeCoapResponse(LwM2mResponseBuilder.java:506)
	at org.eclipse.leshan.server.californium.request.LwM2mResponseBuilder.visit(LwM2mResponseBuilder.java:307)
	at org.eclipse.leshan.core.request.ReadCompositeRequest.accept(ReadCompositeRequest.java:105)
	at org.eclipse.leshan.server.californium.endpoint.ServerCoapMessageTranslator.createLwM2mResponse(ServerCoapMessageTranslator.java:75)
	at org.eclipse.leshan.server.californium.endpoint.CaliforniumServerEndpoint$1.buildResponse(CaliforniumServerEndpoint.java:111)
	at org.eclipse.leshan.core.californium.SyncRequestObserver.waitForResponse(SyncRequestObserver.java:55)
	at org.eclipse.leshan.server.californium.endpoint.CaliforniumServerEndpoint.send(CaliforniumServerEndpoint.java:137)
	at org.eclipse.leshan.server.request.DefaultDownlinkRequestSender.send(DefaultDownlinkRequestSender.java:93)
	at org.eclipse.leshan.server.queue.QueueModeLwM2mRequestSender.send(QueueModeLwM2mRequestSender.java:60)
	at org.eclipse.leshan.server.LeshanServer.send(LeshanServer.java:467)
	at org.eclipse.leshan.server.LeshanServer.send(LeshanServer.java:440)

From : #1502

The equals/hashcode implementation does not really support well inheritance but we should decide what is the right way to follow first : #1504