Helm proxy repositories return 500 instead of 404. Log shows NPE

The fix for NEXUS-25611 has introduced a regression. A helm proxy repository will now return a 500 error response when it should return 404.  

To reproduce, upload a helm package to a hosted helm repository.  I've attached the one I used. 

Create a helm proxy to that repository in a 3.29.1 instance.   Request the tgz with a different version through the proxy, e.g.:

 http://localhost:8083/repository/helm-proxy/mysql-6.14.9.tgz

This will fail with a 500 response.

Check the logs, they will show an NPE:

2020-12-29 11:26:34,833-0600 WARN [qtp1054182676-212] admin org.sonatype.nexus.repository.httpbridge.internal.ViewServlet - Failure servicing: GET /repository/helm-proxy/mysql-6.14.9.tgz
java.lang.NullPointerException: null
at java.net.URI$Parser.parse(URI.java:3068)
at java.net.URI.<init>(URI.java:588)
at java.net.URI.create(URI.java:850)
at java.net.URI.resolve(URI.java:1036)
at org.sonatype.nexus.repository.proxy.ProxyFacetSupport.fetch(ProxyFacetSupport.java:419)
at org.sonatype.nexus.repository.proxy.ProxyFacetSupport.fetch(ProxyFacetSupport.java:408)
at org.sonatype.nexus.repository.proxy.ProxyFacetSupport.doGet(ProxyFacetSupport.java:275)
at org.sonatype.nexus.repository.proxy.ProxyFacetSupport.lambda$1(ProxyFacetSupport.java:251)
at org.sonatype.nexus.common.io.CooperatingFuture.performCall(CooperatingFuture.java:122)
at org.sonatype.nexus.common.io.CooperatingFuture.call(CooperatingFuture.java:64)
at org.sonatype.nexus.common.io.ScopedCooperationFactorySupport$ScopedCooperation.cooperate(ScopedCooperationFactorySupport.java:99)
at org.sonatype.nexus.repository.proxy.ProxyFacetSupport.get(ProxyFacetSupport.java:242)
at org.sonatype.nexus.repository.proxy.ProxyHandler.handle(ProxyHandler.java:52)
at org.sonatype.nexus.repository.view.Context.proceed(Context.java:88)
at org.sonatype.nexus.repository.storage.UnitOfWorkHandler.handle(UnitOfWorkHandler.java:39)
at org.sonatype.nexus.repository.view.Context.proceed(Context.java:88)
at org.sonatype.nexus.repository.view.handlers.ContentHeadersHandler.handle(ContentHeadersHandler.java:46)
at org.sonatype.nexus.repository.view.Context.proceed(Context.java:88)
at org.sonatype.nexus.repository.http.PartialFetchHandler.handle(PartialFetchHandler.java:59)
at org.sonatype.nexus.repository.view.Context.proceed(Context.java:88)
at org.sonatype.nexus.repository.cache.NegativeCacheHandler.handle(NegativeCacheHandler.java:56)
at org.sonatype.nexus.repository.view.Context.proceed(Context.java:88)
at com.sonatype.nexus.clm.internal.orient.FirewallContributedHandler.handle(FirewallContributedHandler.java:101)
at org.sonatype.nexus.repository.view.Context.proceed(Context.java:88)
at com.sonatype.nexus.usertoken.plugin.internal.UserTokenHandler.handle(UserTokenHandler.java:72)
at org.sonatype.nexus.repository.view.Context.proceed(Context.java:88)
at org.sonatype.nexus.repository.view.handlers.HandlerContributor.handle(HandlerContributor.java:67)
at org.sonatype.nexus.repository.view.Context.proceed(Context.java:88)
at org.sonatype.nexus.repository.view.handlers.ExceptionHandler.handle(ExceptionHandler.java:42)