UploadArchive: Correct check for tree-ish not found

Repository#resolve(String) returns null, not zeroId, if it couldn't
resolve.

Coverity-CID: 80898
Change-Id: I29e20c94cb4848241a06e41d78f033e06ae84eba

gerrit-sshd/src/main/java/com/google/gerrit/sshd/commands/UploadArchive.java

@@ -162,7 +162,7 @@ public class UploadArchive extends AbstractGitCommand {
 
       // Find out the object to get from the specified reference and paths
       ObjectId treeId = repo.resolve(options.treeIsh);
-      if (treeId.equals(ObjectId.zeroId())) {
+      if (treeId == null) {
         throw new Failure(4, "fatal: reference not found");
       }