swh-graph: NullPointerException upon (wrong) /walk from cnt to snp

• input: GET http://localhost:5009/walk/swh:1:cnt:000070db8360fe14a0291021bc53e59b1fd0282d/snp
• expected output: empty path (note that i've requested an impossible path, from an existing cnt to a snapshot node, but i have //not// specified ?direction=backward; adding that makes the request works fine)
• actual output: internal server error with the traceback below on the server's stderr:

[qtp1648001170-13] WARN io.javalin.Javalin - Uncaught exception
java.lang.NullPointerException
	at org.softwareheritage.graph.Endpoint.convertNodesToSwhPath(Endpoint.java:59)
	at org.softwareheritage.graph.Endpoint.walk(Endpoint.java:135)
	at org.softwareheritage.graph.App.lambda$main$10(App.java:115)
	at io.javalin.core.security.SecurityUtil.noopAccessManager(SecurityUtil.kt:22)
	at io.javalin.http.JavalinServlet$addHandler$protectedHandler$1.handle(JavalinServlet.kt:139)
	at io.javalin.http.JavalinServlet$service$2$1.invoke(JavalinServlet.kt:45)
	at io.javalin.http.JavalinServlet$service$2$1.invoke(JavalinServlet.kt:25)
	at io.javalin.http.JavalinServlet$service$1.invoke(JavalinServlet.kt:146)
	at io.javalin.http.JavalinServlet$service$2.invoke(JavalinServlet.kt:40)
	at io.javalin.http.JavalinServlet.service(JavalinServlet.kt:98)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
	at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:873)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:542)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:255)
	at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1700)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:255)
	at io.javalin.core.JavalinServer$start$httpHandler$1.doHandle(JavalinServer.kt:47)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:203)
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:480)
	at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1667)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:201)
	at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1247)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:144)
	at org.eclipse.jetty.server.handler.HandlerList.handle(HandlerList.java:61)
	at org.eclipse.jetty.server.handler.StatisticsHandler.handle(StatisticsHandler.java:174)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
	at org.eclipse.jetty.server.Server.handle(Server.java:505)
	at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:370)
	at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:267)
	at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:305)
	at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:103)
	at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:117)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.runTask(EatWhatYouKill.java:333)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:310)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:168)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:126)
	at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:366)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:786)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:743)
	at java.base/java.lang.Thread.run(Thread.java:835)

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Migrated from T1938 (view on Phabricator)